A common aromatherapy myth states that essential oils are inherently safe and will not interfere with any pharmaceutical drugs. While most essential oils are safe with normal diluted topical use, there are a few exceptions.
Caution should be excercised using any topical aromatherapy preparations around drug injection sites or areas of the body where transdermal medications are in use (i.e., estrogen or nicotine patches, etc.).
Drug interactions may be problematic even with simple inhalation of essential oils. For instance, inhalation of EO's high in menthol, such as peppermint, will cause increased lung permeability of nicotine. Menthol also slows the ability to clear nicotine from the body. If you are a smoker, allow several hours to elapse between cigarette use and inhalation of high menthol EO's.
Many oils commonly used for respiratory issues, such as rosemary, eucalyptus, ravintsara and bay laurel are high in 1,8 cineole. Cineole can interfere with metabolism of anaesthesia, and should be avoided (both topically and via inhalation) for at least a week prior to any surgeries to prevent complications. High cineole oils are also contraindicated for those on barbituates, as they induce rapid metabolism of these drugs.
Essential oils which are high in eugenol or methyl salicylate should not be used by persons on anticoagulant drugs. Methyl salicylate can also cause problems for asthmatics or those with aspirin allergies. It is possible for EO's which contain high levels of these phytochemicals to cause problems even upon inhalation and dilute topical application.
Persons with renal or liver disease, compromised immune systems, or those taking multiple medications should consult with a qualified professional aromatherapist before attempting to self-medicate with essential oils.
Other than the examples cited above, most known drug interactions in aromatherapy occur with oral, vaginal or rectal use of essential oils. Although these applications are common in parts of Europe, aromatherapy is seldom administered this way in the United States. Still, I receive occasional reports of inexperienced people who believe it is safe to use EO's internally. Even if you are not taking any medications, please do not attempt to use essential oils internally unless you are under the direct supervision of a qualified aromatherapist or a medical practitioner with aromatherapy training.
Harris, Rhiannon. Drug-Essential Oil Interactions: Risks and Reassurances. Presentation to Alliance of International Aromatherapists, (17 Dec. 2008). Denver, CO.