Essential oils are highly concentrated extracts of the volatile oils from plants. They can cause damage to sensitive parts of the body, especially in young children, and so should be kept out of their reach.
Research has indicated that essential oils may damage the lining of the stomach. In the UK there has been some argument over whether people who are not qualified as doctors are entitled to "prescribe" medications for internal use. The following ruling was adopted by the AOC in July 1998: "The AOC does not permit treatment via oral ingestion of essential oils by qualified aromatherapy practitioners of its member associations unless they are also suitably qualified to the equivalent standard of a medical herbalist or a medical doctor."
The following lists are based on lists issued by the IFA. A few further oils, and explanatory comments have been added.
| Almond (Bitter) | Prunus amygdalis, var. amara | Highly toxic |
| Aniseed | Pimpinella anisum | Addictive, toxic if used long term |
| Arnica | Arnica montana | Toxic |
| Boldo leaf | Peumus boldus | Toxic, skin irritant, convulsive |
| Calamus | Acorus calamus-acorus | Toxic, skin irritant, possible carcinogen |
| Camphor | Cinnamomum camphorus | Toxic, skin irritant, possible carcinogen |
| Cassia | Cinnamomum cassia | Violent skin irritant and sensitiser |
| Cinnamon bark | Cinnamomum zeylanicum | Skin irritant and sensitiser |
| Clove, all parts | Eugenia caryphyllata | Skin irritant and sensitiser |
| Costus | Saussurea lappa | Skin sensitiser |
| Elecampane | Inula helenium | Skin sensitiser |
| Fennel (Bitter) | Foeniculum vulgare | Toxic, can induce fits |
| Horseradish | Cochlearia armorica | Toxic, skin irritant |
| Jaborandi leaf | Pilocarpus jaborandi | Toxic |
| Mugwort (Armoise) | Artemisia vulgaris | Toxic, abortive, convulsive |
| Mustard | Brassica nigra | Toxic, powerful skin irritant |
| Origanum | Origanum vulgare | Skin irritant |
| Origanum (Spanish) | Thymus capitatus | Skin irritant |
| Pennyroyal (European) | Mentha pulegium | Toxic, abortive |
| Pennyroyal (N. American) | Hedeoma pulegiodes | Toxic, abortive |
| Pine (Dwarf) | Pinus pumilio | Skin irritant |
| Rue | Ruta graveolens | Toxic, abortive, skin sensitising |
| Sage | Salvia officinalis | Toxic, abortive, can induce fits |
| Sassafras | Sassafras albidum | Skin irritant, possible carcinogen |
| Sassafras (Brazilian) | Ocotea cymbarum | Skin irritant, possible carcinogen |
| Savin | Juniperus sabina | Toxic, abortive |
| Savory (Summer) | Satureia hortensis | Skin irritant |
| Savory (Winter) | Satureia montana | Skin irritant |
| Southernwood | Artemisia abrotanum | Toxic, probably abortive |
| Tansy | Tanacetum vulgare | Toxic |
| Thuja (Cedarleaf) | Thuja occidentalis | Highly toxic, abortive |
| Thuja Plicata | Thuja plicata | Highly toxic, abortive |
| Wintergreen | Gaultheria procumbens | Toxic |
| Wormseed | Chenopodium anthelminticum | Highly toxic, skin irritant |
| Wormwood | Artemisia absinthium | Toxic, abortive |
| Fennel (Sweet) | Foeniculum vulgare | Can induce fits |
| Hyssop | Hyssopus officinalis | Can induce fits |
| Rosemary | Rosmarinus officinalis | May induce fits in large amounts |
|
(and Wormwood and Sage, which are on the "Not to be used at all" list, above) |
| Basil | Ocymum basilicum | Mildly toxic, possible abortive |
| Birch | Betula alba, B. lenta, B. alleghaniensis | |
| Camomile (Roman) | Anthemis nobilis | |
| Cedarwood | Cedrus atlantica | Chronic toxicity |
| Cinnamon leaf | Cinnamonum zylanicum | Chronic toxicity, skin irritant |
| Clary Sage | Salvia sclarea | |
| Cypress | Cupressus sempervirens | |
| Eucalyptus | Eucalyptus globulus | Chronic toxicity |
| Fennel (sweet) | Foeniculum vulgaris | Chronic toxicity, fits |
| Geranium | Pelargonium asperum x roseum | |
| Hyssop | Hyssopus officinalis | Chronic toxicity, fits |
| Jasmine | Jasminium officinalis, J. grandiflorum | |
| Juniper | Juniperis communis | |
| Lavender | Lavandula officinalis, L. vera, L. angustifolia | emmenagogue |
| Lemon | Citrus lemonum | |
| Marjoram | Origanum majorana | |
| Myrrh | Commiphora myrrha | |
| Nutmeg | Myristica fragrans | |
| Orange (bitter) | Citrus aurantium | |
| Orange (sweet) | Citrus sinensis | |
| Peppermint | Mentha piperata | |
| Rosemary | Rosmarinus officinalis | |
| Tarragon | Artemisia dranunculus | |
| Thyme | Thymus vulgaris |
|
A few of these may be used in some circumstances by experienced aromatherapists |
These oils should be used with caution, and not for more than a few days at any one time. Photosensitisers should not be used on the skin before exposure to sunlight.
| Angelica | Angelica archangelica | Skin sensitiser, photosensitiser |
| Basil | Occimum basilicum | Mildly toxic, possibly abortive |
| Bergamot | Citrus bergamia | Photosensitiser |
| Black Pepper | Piper nigrum | Skin irritant |
| Cedarwood | Cedrus atlantica | Chronic toxicity |
| Cinnamon Leaf | Cinnamomum Zeylanicum | Chronic toxicity, skin irritant |
| Citronella | Cymbopogon nardus | Skin irritant |
| Eucalyptus | Eucalyptus globulus | Chronic toxicity |
| Fennel (Sweet) | Foeniculum vulgare | Chronic toxicity, fits |
| Ginger | Zingiber officinalis | Skin irritant |
| Hyssop | Hyssopus officinalis | Chronic toxicity, fits |
| Lemon | Citrus limonum | Chronic toxicity, skin irritant, photosensitiser |
| Lemongrass | Cymbopogon citratus | Skin irritant |
| Lemon verbena | Lippia citriodora | Skin irritant |
| Melissa | Melissa officinalis | Skin irritant |
| Orange (bitter) | Citrus aurantium | Chronic toxicity, skin irritant, photosensitiser |
| Nutmeg | Myristica fragrans | Chronic toxicity, skin irritant |
| Peppermint | Mentha piperita | Skin irritant |
| Thyme | Thymus vulgaris | Chronic toxicity |
| Verbena | Lippia citriodora | Skin irritant |