Active Therapeutic Agents in Medicinal Plants: an indepth look at the chemicals in medicinal plants.

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Therapeutically active ingredients in Medicinal Plants

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Therapeutic agents, or therapeutic components/ingredients of medicinal plants come in many forms and have varying degrees of strength. Alkaloids, for example, can be both very therapeutic as well as potentially deadly. Just because a substance is natural does not mean it can do you no harm - think of arsenic for example.

Therapeutic Agents






























General Overview

The therapeutic agents found in medicinal plants are products of plant metabolism. They are produced and stored in various plant tissues including leaves, stems, flowers, roots, bark, seeds and fruits.

Therapeutic agents are generally separated into toxic and non-toxic classifications.

Symptoms of mild poisoning include:

  • nausea and vomiting
  • diarrhoea
  • abdominal cramps
  • headaches and vertigo

Aromatic species of plants (i.e. those containing essential oils) are generally considered non-toxic, but may impair the function of various body organs if used in excessive quantities or over longer than prescribed periods of time.

Therapeutic Constituents and Their Properties


Alkaloids are distinguished by a distinctive nitrogen compound found in their chemical structure. Alkaloids can occur in all parts of the plant and are in greatest concentrations just before the flowering period.

Despite being the most significant therapeutic substances known, they can also have marked toxic effects when taken in high dosages or for prolonged periods of time. The method used to prepare a botanical medicine from a herb containing alkaloids is a significant factor that influences the potential level of toxicity it will have.

Alkaloids are destroyed by high temperatures and are relatively insoluble in water: thus, water infusions or decoctions are traditionally the preferred way to dispense alkaloid-containing herbs. As alkaloids mix readily with ether, chloroform and alcohol, medicinal preparations based on these must be standardised and dosage recommendations strictly adhered to.


Plant Source Alkaloid
Papaver somniferum (Opium) morphine
Alstonia constricta (Australian quinine) quinine
Aconitum napellus (Monkshood) coniine
Nicotiniana tabacum (Tobacco) nicotine
Atropa belladonna (Deadly nightshade) atropine
Colchicum autumnale (Autumn crocus) colchicine
Hyoscyamus niger (Henbane) hyoscyamine


Atropine and hyoscyamine are smooth muscle relaxants. Morphine, narcotine, papaverine, codine, thebaine, alkaloids found in the opium poppy (Papaver somniferum), are known to relieve pain, relax cramps and suppress the cough reflex.


Glycosides are naturally occurring chemical compounds composed of a sugar portion attached to non-sugar portion by a special chemical bond that requires specific enzymes to be broken.
They have a pronounced physiological action and are potentially toxic to man.
Glycoside concentration in a plant depends upon the age of the plant or  its ecology (i.e where it was grown).

The most significant forms in this group, cardiac glycosides, anthra-quinone glycosides, thuicyanate glycosides and phenolic glycosides, are discussed below.


Cardiac glycosides have a very strong action in the body and even in small doses they can be extremely poisonous.

Therapeutically, they improve heart contraction and function and are traditionally used  in heart failure. Cardiac glycosides tend to accumulate in the system as their elimination from the body is slow and dosage recommendation must be strictly adhered to.

Examples of medicinal plants that contain cardiac glycosides are Digitalis lanata (Foxglove - used extensively in orthodox medicine as a heart medication), Adonis vernalis (False Hellibore) and Convullaria majalis (Lily of the Valley).


Anthra-quinone glycosides are found in medicinal plants that traditionally have been used as laxatives. They are mildly poisonous in large doses, causing nausea, headache and diarrhoea but are not fatally toxic.

Laxatives generally cause bowel laziness and disturb the body's natural rhythm, but the plants containing Anthra-quinone Glycosides do not do this.

Examples of plants containing Anthra-quinone glycosides are Rheum palmatum (Rhubarb) and Rubia tinctorium (Madder Root).


Thuicyanate glycosides are characterised by a sulphur molecule in their chemical structure. They are very unstable chemically and are easily affected by temperature.
Thuicyanate glycosides have an irritant and disinfectant action on body organs. They also improve blood supply to tissues and can produce local inflammation externally.
An example of a medicinal plant that contains Thuicyanate glycosides is Brassicas nigra (Black Mustard).


Phenolic glycosides are only slightly poisonous if taken in excessive doses. Therapeutically they have an affect on the kidneys and liver primarily being disinfectant, anti-inflammatory and diuretic. An example of a medicinal plant that contains phenolic glycosides is Arctostaphylos uva-ursi (Bearberry).


Saponins have a similar chemical structure to glycosides but produce a soapy lather when shaken with water.

Some saponins can be strongly physiologically active, breaking up red blood cells (haemolysis) and can therefore be potent poisons. Many are marked irritants.

Saponins also possess beneficial therapeutic properties including having an expectorant effect on the upper respiratory tract (sinuses, nose, throat).

An example of a medicinal plant that contains saponins is Arnica montana (Arnica).


Bitter principles, or ‘bitters’ as they are more commonly known, are non poisonous substances that occur widely in medicinal plants. They are strongly bitter, but not unpleasant in taste and are often referred to as STOMACHICS meaning that they improve digestion by relaxing and toning. For this reason they are used traditionally for people undergoing convalescence after illness and to improve deficient appetite and the flow and quality of gastric juices in conditions of poor health. When used for these purposes they are administered 10 to 15 minutes before meals.

Therapeutically, bitters produce a state of hyperaemia and motor stimulation of the gastric mucosa, stimulate the taste buds and excite the flow of gastric juices. They also have an antiseptic action in the gastro-intestinal tract.

Examples of medicinal plants containing bitter principles are Gentiana lutea (Gentian) and Agrimonia eupatoria (Agrimony) and all plants in the Gentianaceae family.


Tannins and astringents are also commonly found in medicinal plants, often occurring together, and are non-toxic. They are soluble in water and alcohol, oxidise and turn dark on exposure to air and tend to lose their therapeutic properties if stored for long periods of time.

Tannins and astringents are have pronounced physiological actions both internally and externally.

When applied to the skin or when ingested they produce localised tissue reactions, some of which are listed below:

  • Produce visible contraction of tissue
  • Blanch and wrinkle mucous membrane.
  • Diminish exudation (discharge)
  • Produce slight precipitation of proteins on surface layers of tissue (drying effect)
  • Reduce inflammation
  • Produce a thin protective film on surface
  • Check migration of leucocytes and pus formation
  • Vasoconstriction (check bleeding)

These reactions make astringents and tannins useful in the following therapeutic applications:
  • To protect inflamed mucous surfaces
  • To reduce excessive secretions
  • To relieve diarrhoea
  • To reduce swelling, inflammation and secretion
  • To prevent bleeding from small wounds, and in the treatment of conditions such as haemorrhoids, varicose veins, minor burns, hay fever and diarrhoea.

Examples of medicinal plants that contain astringents and tannins include Hamamelis virginiana (Witchhazel) and those in the Rosaceae and Gentianaceae families.


Mucilage is a non toxic substance that is composed of chains of chemically linked sugars (polysaccharides). It is destroyed by prolonged heating and alcohol but is partly soluble in water: which causes it to swell and form gel.

Therapeutically, mucilage has been shown to be a useful laxative that works by increasing the intestinal bulk and favouring peristaltic contractions. Mucilage also serves to protect and coat the mucosa of the intestines against irritants, being soothing in cases of diarrhoea.

Examples of medicinal plants that contain mucilage are Linum usitatissimum (Linseed/Flax), Ulmus fulva (Slippery Elm Bark) and Malva officinale (Mallow).


The chemical structure of essential oils is based upon terpenes, terpene compounds and terpene derivatives. These compounds are stored in special glands in the plant tissues and are known to have multiple effects on human physiology when used in therapeutic doses, some examples of which are listed below.

1) Antiseptic: inhibits growth of pathogenic microbes

Some examples of essential oils with antiseptic properties are Thymus vulgaris (Thyme Oil), Allium sativum (Garlic Oil), Allium cepa (Onion oil) and Lavendula officinale (Lavender Oil).

2) Insect repellent, anthelmintic (anti parasitic).

Some examples of essential oils with insect repellent and anti-parasitic properties are Anisum vulgare (Aniseed Oil), Pogostemon patchouli (Patchouli Oil).

3) Counter-irritant:

Causes inflammation and swelling of skin and increases blood supply to tissues. An example of an essential oil which has this counter-irritant effect is Brassica nigra (Black Mustard Oil)

4) Anti-inflammatory :

Reduces inflammation and counteracts fever. Some examples of essential oils with anti-inflammatory properties are Matricaria chamomila (Chamomile Oil) and Mentha piperita (Peppermint Oil).

5) Anaesthetic effects:

Causes a feeling of numbness. Some examples of essential oils with anaesthetic properties are Eugenia caryophyllata (Clove Oil) and Mentha piperita (Peppermint Oil).

6) Cardiac vascular tonic:

Stimulation of heart muscle and circulation. An example of an essential oil with this property is Cinnamomum camphora (Camphor Oil).

7) Sedative:

Relaxes and calms the nervous system or part of it. Some examples of essential oils with sedative properties are Valeriana officinalis (Valerian Oil), Lippia citriodora (Melissa Oil), Mentha piperita (Peppermint Oil) and Matricaria chamomila.

8) Stomachic:

Affects the smooth muscle of gastro-intestinal track including the liver and gall bladder; stimulates the flow of gastric secretions and bile; stimulates appetite; alleviates digestive disorders. Some examples of essential oils with stomachic properties are Citrus aurantium (Bergamot Oil) and Origanum marjorana (Marjoram Oil).

9) Diuretic and antiseptic to the urinary tract.

Some examples of essential oils with these properties are Carum sativum (Parsley Seed Oil) and Juniperus communis (Juniper Oil).



Resins and gum resins are more or less solid amorphous products of a complex chemical nature. Chemically resins are complex mixtures of resin acids, resin alcohols (resinols) and resin phenols (resino-tannols), esters and resenes. They are insoluble in water and petroleum spirit but will dissolve in alcohol, chloroform and ether. On heating they soften and melt. Most resins and gum resins are produced by the plant as a response to injury.

Therapeutically they have soothing and anti-inflammatory properties. Examples of resins abd gum-resins are Commiphora myrrha (Myrrh resin), Myroxylon pereirae (Balsam of Peru) and Myroxylon Balsamum (Balsam Tolu).


Acknowledgement: The following information is largely sourced from The Essential Oil Safety Data Manual, written and researched by Robert Tisserand (1990).

Pure essential oils are composed of naturally occurring chemical compounds manufactured within the plants from which they are extracted. The major components of essential oils, carbon-based organic molecules, have been known for many years and since the introduction of gas liquid chromatography, very small and even some trace constituents can now be detected and identified.

Different essential oils may contain the same constituents in differing proportions. For example, cineol is a chemical compound found in many oils and in some oils is a principal constituent (P.C.) whilst in others it is a minor or a trace constituent.

Cineol is the P.C. (principal constituent) of eucalyptus oil and usually occurs at around 80%. However, in the case of mandarin oil, cineol has been detected at 0.002% - 40,000 times less than the amount found in eucalyptus oil.

Most essential oils contain somewhere between 10 and 200 components and some contain only two or three major components.

The table below lists some of the major chemical groups and their general therapeutic activities found in commonly used essential oils.



Phenylpropane derivatives consist of an aromatic phenyl ring with a 3 Carbon side chain that may then be modified by various other groups that attach to it. This chemical group has the following characteristics:

  • Very reactive
  • Skin irritant (potential)
  • Fungicidal
  • Antiseptic
  • Anti-carcinogenic
  • Local anaesthetic
  • Distinctly sweet character in fragrance
  • Potentially neurotoxic (affect Central Nervous System).

Examples are shown in the table below.



Clove Oil Eugenol Antiseptic
Cinnamon Oil Cinnamic aldehyde Stimulant and Skin irritant
Aniseed Oil Anethol Increased secretion
Basil Oil
Tarragon Oil

Parsley Oil Nutmeg Oil
Sassafras Oil
Safrol, Myristicin & Apiol
Nervous System stimulant

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Terpenes are the most common chemical groups found in essential oils. In addition, they have the broadest range of therapeutic properties with relatively little to no toxicity associated with them.


Terpene hydrocarbons such as Limonene, Pinene, Camphene and Myrcene are known to have antiviral activity and are thought to be skin and mucous membrane irritants. These are found in the following essential oils: Lemon, Orange, Bergamot, Black Pepper, Pine, Turpentine, Nutmeg, Mastick, Angelica.

The basic terpene molecule is a monoterpene hydrocarbon consisting of 10 carbon atoms. This basic molecular structure is modified by the addition of extra carbon chains.


10 Carbon atoms in the basic molecule and form the primary constituents of many essential oils.


20 Carbon atoms in the basic molecule and are less commonly found in essential oils.


15 Carbon atoms in the basic molecule and are commonly encountered in essential oils. The sesquiterpene molecule is large, the molecular function is more complex and the influence of a functional group in a sesquiterpene is less than that of the terpene part.

Sesquiterpenes are mainly found in oils distilled from roots and woods and plants of the Asteraceae family (Compositae).

Limited research data is available but these are known to be immune system stimulants. Examples of sesquiterpenes are Chamazulene, Bisabolol, Santalol, Zingiberol, Carotol Caryophyllen and Farnesol.


A terpene alcohol molecule consists of a Terpene portion and an alcohol portion.
These are very useful in Aromatherapy. They have a strong reactive power and are very fluid. They are versatile with low toxicity, strong antiseptic activity and a positively energising and uplifting effect.

Examples of som eterpene alcohols and the essential oils in which they are found are:

  • Linalol: Lavender, Rosewood, Petitgrain, Neroli, Coriander
  • Citronellol: Rose, Geranium (antiviral)
  • Geraniol: Palmarosa
  • Alpha Terpineol: Eucalyptus, Niaouli
  • Terpineol 4: Cajuput


When terpenes and sesquiterpenes are modified by the addition of a functional group, the terpene or sesquiterpene is then called a terpenoid or sesquiterpenoid compound.

Terpenoids  and sesquiterpeneoids have 30 and 40 Carbon atoms respectively. They are not found in essential oils obtained by steam distillation as their molecules are large and cannot be carried in water vapour. They are found in association with tannins, flavonoids, carotenoids, polysaccharides, steroids and hormones.

Some of the main therapeutic activities of terpenes & terpene compounds are listed below.

Antibiotic (antibacterial, antifungal antiseptic, antiviral)
Antiinflammatory, antiphlogistic
Antitumour (antiblastic, anticarcinogenic, cytotoxic)

Choleretic, cholagogue
Phyto-hormones (growth regulating)
Vitamin source



Oxides are formed by an Oxygen atom linking 2 Carbon atoms in a ring structure. These tend to be found in oils with expectorant properties, for example cineol/eucalyptole (Eucalyptus globulus) and linalol oxide (Hyssopus officinale).


Phenols are composed of a benzene ring with an hydroxyl side chain. They are very chemically active and have bactericidal properties.

They are strong stimulants to the liver and kidneys and should always be used in low concentrations. Some phenols ease or reduce the flow of mucous in the Upper Respiratory Tract (nose, throat, sinuses), some are cytophylactic and therefore useful in skin care and some are neurotoxic if taken in large doses internally [e,g. Thuja, Pennyroyal, Mugwort, Sage].
Examples of some common phenols areThymol, Carvacrol and Eugenol.


Monoterpenoid Aldehydes are sedative and antiseptic and are commonly found in the essential oils of Melissa, Lemongrass, Citronella, Eucalyptus citriodora.
Examples of aldehydes include Citral, Citronellal, Neral and Geranial.


Esters have antifungal properties and are sedative and spasmolytic. They are recognised by their sweet, fragrant and fruity aromas. Only a few essential oils contain esters which are important in providing finer notes. Esters are found in Roman Chamomile, Lavender, Bergamot and Clary Sage.

Examples of common esters are Linalyl acetate, Geranyl acetate, Bornyl acetate and Methyl Salicylates.


Ketones are very physiologically active, being cytophylactic and mucolytic. If used internally they arepotentially neurotoxic and can be irritant to kidneys and liver. Some examples of common ketones are Thujone, Pulegone, Pinocamphone and Carvone.

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Now that you know the main chemical compounds found in essential oils, it is logical and vital that you know and appreciate the potential toxicity of essential oils. “Because essential oils are natural substances does not automatically guarantee their safety" (Tisserand, 1991). It is a myth that 'natural substances' are not harmful! Many poisonous and deadly things are found in nature! However, awareness of potential risks is the best way to avoid mishaps and the following information is offered with this in mind.

Plants containing essential oils have a long history of use by Herbalists. It is often the case that the herb may be safe to use but its isolated essential oil may not be and it has been estimated that the therapeutic properties of essential oils can be up to 70 times more concentrated than the herbs from which they come. It therefore becomes immediately obvious that essential oils must be used with utmost care and respect. For practitioners of herbal medicine and aromatherapy, a sound knowledge of their chemical make-up and physiological activity is imperative.

There are several levels of Toxicity based upon the LD50 (Lethal Dose 50%) Scale. Research into the chemistry and pharmacological activity of essential oil components over the past 20 years have revealed the concentrations and dosages at which each essential oil becomes toxic (i.e. the LD 50). Most of this research data is based upon the results of animal experimentation with the essential oils tested primarily used in an undiluted form rather than a diluted form. Aromatherapists use essential oils in the diluted form 99.9% of the time!

It is not only an unlucky patient who may experience the negative side effects of a high dose of essential oils. Therapists who work with oils constantly are also at risk if they do not observe the guidelines for safe handling of essential oils.

The categories associated with Essential Oil Health Hazards and Toxicity are:



Also known as poisoning, a toxicity reaction can be fatal at a certain level, whether the oil is taken ORALLY or ABSORBED through the skin. Toxicity is dose dependent - the greater the amount of oil ingested or applied to the body, the greater the risk of poisoning occurring.


Irritation may occur on the skin or the mucous membranes depending on the site and mode of application. Chronic irritation can be associated with CARCINOGENESIS (cancer formation). Irritation is also dose-dependent.


Also referred to as an "allergic reaction". In some individuals even a very small amount of a substance can produce an allergic reaction. Some essential oils are classified as common irritants because of their chemical constituents.

On the other hand, some people have what is termed "idiosyncratic allergic reactions" to essential oils: that is, the oil may have no innate allergy producing components but the individual has an allergic reaction to it.

Toxicity, Irritation and Sensitisation may be ACUTE or CHRONIC in character.

An ACUTE event is one that occurs suddenly after one dose or application of an essential oil. A CHRONIC event is one that occurs over a period of time and is usually associated with repeated irritation of tissues.

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This occurs as a result of a single dose given orally, producing a toxic effect in the body. The potential for an essential oil to produce an acute oral toxicity reaction is measured by calculating the number of grams of essential oil required to induce death per kilogram of body weight (gms/kg of body weight). This is the criterion upon which the LD50 TEST is based.

For general use, the LD50 Scale is divided into 4 categories:

  • A: Very Potent if less than 0.02gm/kg
  • B: Potent from 0.02 to 0.2gm/kg
  • C : Marginal from 0.2 to 2.0gm/kg
  • D: Safe if 2.0gm/kg or greater

Examples of the LD50 values of some essential oils are shown below in their relevant groups and have been calculated on the LD50 values that would apply to an adult weighing 70 kg.


The Essential Oils below are considered to have the potential to be very lethal poisons and thus belong to Group A. Rue, Savin and Wintergreen are also included in Group A because of the principle chemical constituents they contain. There is no actual toxicity data available for these three oils.
None of the Group A oils are used in Wildcrafted Herbal Products.


Boldo Leaf
Pennyroyal (American)
Pennyroyal (European)
Bitter Almond
(in gms/mls)

(in gms/mls)




The Essential Oils below belong to the Group B Toxicity Rating, meaning that they are 'marginally' lethal poisons. (On the basis of their major chemical constituents, Buchu, Thyme, Dalmatian Sage and Parsley Seed are also included in this group). Essential oils from this group that are used in the Wildcrafted Product Range are highlighted (please note Myrrh is used in tincture form).


(in gms/mls)
LD 50
(in gms/mls)
Cornmint 1.25 87.5g
Savory (summer & winter) 1.37 95.9g
Clove Leaf 1.37 95.9g
Basil 1.40 98.0g
Hyssop 1.40 98.0g
Bitter Almond (free from Prussic acid) 1.49 104.3g
Sassafras (Brazilian) 1.58 110.6g
Myrrh 1.65 115.5g
Birch (Sweet) 1.70 119.0g
Bay Leaf (West Indian) 1.80 126.0g
Origanum 1.85 129.5g
Sassafras 1.90 133.0g
Tarragon 1.90 133.0g
Marjoram (Sweet) 2.24 156.8g



Some essential oils are considered non-toxic but still must be used with care because of the chemical constituents they contain. These essential oils are Group C and are listed below. Highlighted oils are those used in the Wildcrafted Product Range.


Aniseed Star Aniseed
Bay Leaf (laurel) Cajuput
Camphor (brown) Camphor (yellow)
Caraway Cassia
Cinnamon Bark Cinnamon Leaf
Clove Bud Clove stem
Coriander Costus
Cumin Dill Seed
Dill weed Elemi
Eucalyptus Fennel (bitter)
Fennel(Sweet) Ho Leaf
Lavender (Spike) Lovage
Mace Marjoram (Sweet)
Neroli Nutmeg
Peru Balsam Pimento Leaf
Rosewood Rue
Dalmatian Sage Snakeroot
Taget Thyme



Group D essential oils are considered to be very safe and huge quantities of these essential oils would have to be ingested or applied to the skin before toxicity would occur. These oils are listed below. Highlighted oils are those used in the Wildcrafted Product Range.


Abies alba (cone) Abies alba(needles)
Angelica Root and Seed Benzoin resinoid
Bergamot Birch Tar
Cabreuva Cade
Camphor (White) Cananga
Cardamom Carrot Seed
Cascarilla Cedarwood (Atlas Texas Virginian)
Celery Seed Chamomile (German and Roman)
Citronella Clary Sage
Copaiba Cubeb
Cypress Davana
Everlasting Fir Needle (Canadian & Siberian)
Galbanum Geranium (Algerian Reunion)
Ginger Grapefruit
Guaiacwood Gurjun
Hibawood Jasmine (absolute)
Juniper Labdanum
Lavandin Lavender
Lemon Lemongrass
Lime Linaloe
Litsea Cubeba Marjoram (Spanish)
Orange (Bitter and Sweet) Palmarosa
Patchouli Pepper (Black)
Petitgrain Pine (Scotch Dwarf)
Rose (Otto Absolute) Rosemary
Sage (Spanish) Sandalwood
Schinus molle Spearmint
Tangerine Valerian
Vetiver Ylang-Ylang



General signs of toxicity have already been discussed. However, in relation to the LD50 Scale, the following are ALSO considered signs of toxicity if the essential oil produces one or more of these conditions: abortion, convulsion, vermifuge (expels worms).


Most essential oils can be absorbed through the skin. Acute dermal toxicity occurs where an essential oil with potentially high toxicity is absorbed through the skin and/or where it is applied on a larger area of skin. The LD50 Values for these oils are determined from skin tests made on rabbits.

There appears to be a group of major chemical constituents that are implicated in acute dermal toxicity. These major constituents are listed below together with the essential oils that contain them in significant amounts


Ascaridole Eugenol Thymol/Carvacrol Cinnamic Aldehyde
Boldo Leaf Clove Leaf Origanum Cassia
Wormseed Savoury
Cinnamon Bark



This is due to repeated, low level doses given over weeks or months and generally manifests as organ damage of the liver and kidneys.

Chronic toxicity may occur as a result of dermal application, but most often is related to oral ingestion.

The scale for measuring Chronic Toxicity is based upon grams of essential oil per kilogram of body weight per day and is called the MAXIMUM TOLERATED DOSE (MTD). The MTD calculates the highest maximum tolerated dose that can be administered before any noticeable injury is caused to an experimental animal.

The MTD is equivalent, in principle, to the LD50 value of the essential oil and can be reasonably accurately calculated from the LD50 Value by dividing the LD50 value by 6.
Below are some examples of the MTD of 7 essential oils.


Actual MTD
gm/65kg Human
LD50 /MTD Ratio
Mustard 0.15 0.05 3.25 3.0
Sage 2.52 0.4 26.0 6.3
Clove 3.73 0.7 45.5 5.3
Cassia 5.20 0.75 48.8 6.9
Chamomile 8.56 0.8 52.0 10.7
Angelica 11.56 1.5 98.0 7.7
Valerian 15.00 2.3 150.0 6.5


In the above table, the MTD Values are based upon LD50 doses that were administered orally to both mice and rabbits over an 8 week period without noticeable organ damage occurring. The MTD values for a 65kg human are also shown. The column headed “LD50 /MTD Ratio” shows the relationship between these two dosages and demonstrates that the LD50  is consistently higher than the actual Maximum Tolerated Dose.


Many of the herbs and essential oils that are classified as abortion producing have a traditional use in regulating menstruation and not actually for causing abortion, because of the very high potential risks. Over-dosage of such substances produces toxicity symptoms only, such as severe diarrhoea and vomiting, that by reflex action may produce abortion. There are several reported cases of death due to use of abortifacient oils and these have been due to the ingestion of lethal doses.

However, as some women are more susceptible to abortion and miscarriage than others, there is a special category of oils which should be AVOIDED IN PREGNANCY. These oils are listed below.


Pennyroyal (American and European)


Bay Leaf (West Indian)
Clove Leaf and Stem
Bitter Almond (free from prussic acid)
Birch (sweet)
Thyme (wild)
Marjoram (sweet).



In terms of their effects on the mucous membranes, essential oils are classed as Severe (Grade A), Moderate (Grade B), Mild (Grade C) and Non-Irritant (Grade D). It is mostly those essential oils with a high PHENOL content that produce mucous membrane irritation and rarely, some TERPENES may be a problem.

Primary signs and symptoms of mucous membrane irritation are:

  • erythema (redness)
  • ulceration
  • punctate haemorrhages (pin-point bleeding)

Mucous membrane irritation is a DILUTION-DEPENDENT hazard. In other words, the more diluted the essential oil is the less the risk of mucous membrane irritation. For example, a Grade A irritant becomes a Grade B irritant when diluted to 20% and a Grade B irritant becomes Grade C when diluted to 5%.

It was once thought that all essential oils containing terpenes were mucous membrane irritants, but it is now known that this is not the case. Many essential oils containing terpenes are DETERPENATED in order to reduce this potential problem. However, this is not always the end result, especially in the case of those essential oils that contain phenols where reducing the Terpene content tends to cause an imbalance in the chemical constituents and increases the relative percentage of phenols in the oil. Robert Tisserand, in his "Safety Data Manual" reports his own experience of increased mucous membrane irritation in those deterpenated oils containing phenols.

The main chemical constituents of essential oils that are known to produce mucous membrane irritation are Eugenol, Thymol and Carvacrol, Methyl Chavicol and Cinnamic Aldehyde.

Below are lists of Group A and Group B Mucous Membrane Irritants.

Mustard Cassia
Horseradish Cinnamon Bark and Leaf
Thyme Clove Bud
Origanum Allspice (pimento berry)
Thyme (wild) Bay Leaf (West Indies)
Pimento leaf Caraway
Clove stem and leaf Peppermint
Savoury Spearmint



Most of the tests conducted to determine levels of skin irritation due to Essential Oils have been performed on animals and therefore, may not be relevant to humans e.g. Sandalwood is irritant to rabbits' skin, slightly irritant to mice skin, but is non-irritant to human skin. Furthermore, these tests were performed using undiluted oils on animals, but with diluted oils on humans, so the data is not quite valid. However, it may be useful as a guideline.

Listed below are those constituents thought to be primarily responsible for Dermal Irritation and some of the essential oils that contain these constituents.


Savoury Clove Bud, Stem, Leaf Cassia
Thyme Cinnamon Leaf Cinnamon Bark
Origanum Pimento Leaf
Other Phenols Other Aldehydes Allyl Isothiocyanate
Parsley Seed (Apiol) Bitter Almond FFPA Horseradish
Sweet Fennel (Anethol) (Benzaldehyde) Mustard
Litsea cubeba (Citral)

Essential Oils such as Fir Needle (Siberian), Dwarf Pine, Rue and Wintergreen are also considered to be potential Dermal Irritants.

Dilutions of essential oils for use in Aromatherapy are recommeded to range from 1% to 20% depending upon the essential oil being used.


It is known that cancer can develop as a result of chronic irritation from many factors. In the case of essential oils, two specific phenols are thought to be the prime causative agents. These phenols are ASARONE AND SAFROL.

In the tests carried out on animals, liver tumours, both malignant and benign, developed in 46 out of 67 mice (69%). This was after oral administration of Safrol for a period of two years.

Five groups of rats were used:

  • Group one - no Safrol;
  • Group two - 100 ppm* Safrol
  • Group three - 500 ppm Safrol
  • Group four  - 1,000 ppm  Safrol
  • Group five - 5,000 ppm Safrol

It is interesting to note that the rats in Groups 1, 2 and 3 all developed minimal liver damage.

Benign tumours developed in 8/46 (17.4%) of Group 4 rats, and in 5/47 (10.6%) of Group 5 rats. Group 5 rats also developed malignant tumours in 14/47 (29.8%) of the study animals.

Those essential oils that contain high levels of Safrol are:

  • Sassafras: 93% Safrol
  • Camphor (brown  & yellow only): up to 80% Safrol
  • Calamus: 45-80% asarone

Studies have led researchers to believe that Calamus oil is almost twice as carcinogenic as Sassafras oil.


This is an allergic reaction brought on by application of the essential oil to the skin and occasionally through inhalation. Repeated applications of the oil to "sensitised" individuals will usually produce more and more violent allergic reactions.

General signs and symptoms of Dermal sensitisation are:

  • Rash
  • Redness
  • Irritation/itching/burning
  • Slight blistering
  • Contact Dermatitis

Sometimes, the phenomenon of CROSS-SENSITISATION occurs. This is where a substance SIMILAR TO the substance to which a person is sensitive produces an allergic reaction.

According to Robert Tisserand, there are only five essential oils to which "Normal" individuals will have an allergic reaction. These are Costus, Elecampagne and Cinnamon Bark which are considered to be extreme and Bitter Fennel and Cassia which are considered to be moderate.

Common cross-sensitisers are Benzoin resinoid with Peru Balsam and Bay leaf (laurel) with Costus oil.

The issue of allergic reactions to essential oils is open to debate, as different individuals can have widely differing propensity for allergic reactions. For this reason it is wise to think about the type of allergy you may have before using essential oils. If you do suffer from allergies and are concerned about the possibility of an allergic reaction to essential oils, consult a trained Aromatherapist who could perform a "Skin Patch Test" of the oils to be used.


Photosensitisation occurs when an essential oil applied to the skin, causes a skin reaction when exposed to Ultra Violet light. This may be sunlight or artificial UV.

There are three categories of photosensitisation:

  • Phototoxicity (exaggerated sunburn),
  • Photoallergy (dermatitis),
  • Photocarcinogenesis (inconclusive evidence).

The following list of oils are considered to be PHOTOTOXIC and should not be used in sun-tanning preparations or moisturisers if you go into the sun for extended periods.

a) Rutaceae Family

  • Bergamot (expressed)
  • Lemon (expressed)
  • Lime (expressed)
  • Bitter Orange (expressed)
  • Rue


b) Umbelliferae Family

  • Angelica Root
  • Cumin
  • Opoponax


c) Verbenaceae Family

  • Verbena



This is a tendency to allergic reactions to substances that are not normally considered to be allergens. This reaction can be induced by any mode of application and is not a pre-determinable reaction.



Having acknowledged the possible hazards associated with essential oils, it is, nevertheless, not valid to assume that an Essential Oil will be toxic on the basis of its chemical constituents alone. We must remember the principle of SYNERGISM in which the combined actions of all the constituents making up the essential oil may either enhance or dampen the effects of the active principles.


A contra-indication is a sign, symptom or medical condition that prohibits the use of a particular essential oil; for example, Rosemary oil is contra-indicated for use where a person suffers from hypertension (high blood pressure). In this case, the contra-indications relate to the areas of potential toxicity, irritation and sensitisation discussed above.

Robert Tisserand's "Safety Data Manual" has eight categories of CONTRAINDICATIONS, which are listed below.


Bitter Almond
Boldo Leaf
Pennyroyal (European)
Pennyroyal (American)
Sassafras (Brazilian)
Camphor (Brown)
Camphor (Yellow)



Oils Which Should Be Used With Caution

  • Basil
  • Cornmint
  • Ajowan
  • Clove Leaf
  • Clove Stem
  • Pimento Leaf
  • Origanum
  • Sage
  • Savoury
  • Thyme
  • Thyme (wild)
  • Benzoin (resinoid)
  • Jasmine (absolute)
  • Cabbage Rose (absolute)
  • Violet Leaf (absolute)



Oils Which Should Not Be Used On The Skin At All
All oils listed in Category 1 plus:

  • Mustard
  • Horseradish
  • Cinnamon Bark
  • Cassia
  • Clove Leaf
  • Origanum
  • Savory
  • Boldo Leaf
  • Wormseed
  • Costus
  • Elecampagne
  • Clove Stem
  • Clove Bud
  • Dwarf Pine
  • Bitter Fennel

NB: These oils are listed in descending order from most to least hazardous.


Oils Which Should Be Used With Caution On The Skin

Dermal Toxicity B: Caraway

  • Possible Sensitisation: Aniseed, Cade, Ylang-Ylang
  • Photosensitisation: Angelica Root, Bergamot, Cumin, Grapefruit, Lemon, Lime, Mandarin,
  • Opononax, Orange, Rue, Tangerine, Verbena.


Oils To Avoid In Allergic Persons, Dermatitis and Inflammation

  • Aniseed
  • Bay Leaf (laurel)
  • Benzoin (resinoid)
  • Cade
  • Citronella
  • Geranium (Reunion)
  • Ginger
  • Jasmine
  • Litsea cubeba
  • Pine (dwarf)
  • Pine (Scotch)
  • Terebinth
  • Ylang-Ylang



Oils Which Produce Mucous Membrane Irritation:

  • Origanum
  • Clove Leaf
  • Clove Stem
  • Horseradish
  • Mustard
  • Pimento Leaf
  • Savoury
  • Thyme
  • Thyme (wild)

These oils should be used with great caution and in dilution when used in inhalations, douchesand enemas.

CATEGORY SEVEN: Avoid in Pregnancy


Savin Rue
Mugwort Thuja
Tansy Sassafras
Bitter Almond (ffpa) Basil
Bay Leaf (West Indian) Birch (sweet)
Buchu and Clove (leaf and stem) Cornmint
Hyssop Marjoram (sweet)
Myrrh Origanum
Sage Savory
Tarragon Thyme
Wild Thyme.



General Precautions

Essential oils

  • should not be used UNDILUTED over a large area of skin
  • should not be used directly on eyes to treat eye conditions
  • if used in excess, can lead to headaches, nausea and a feeling of unease.


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