Some Words of Caution
When used intelligently, essential oils are far less likely to cause side effects than most over-the-counter drugs. But experts still advise proceeding with caution. In general, people with fair or freckled skin are more likely to experience skin irritation from essential oils, says Los Angeles aromatic consultant John Steele.
He advises all first-time users to perform a simple skin test to avoid allergic reactions: Place a drop of the oil on a cotton swab and apply it to the inside of the wrist or to the inner elbow. Cover with a bandage and don't wash the area for 24 hours. If no itching or redness occurs, the oil should be safe for external use.
Pregnant women should take particular care in using essential oils. The essential oils calamus, mugwort, pennyroyal, sage and wintergreen can induce miscarriage when taken internally, but even inhalation and topical application are strongly discouraged. Basil, hyssop, myrrh, marjoram and thyme can also cause adverse reactions and should be avoided as well.