A study of the changes in the bioactivity of essential oils used singly and as mixtures in aromatherapy.

The assumption by aromatherapists that essential oils are more beneficial when applied as mixes of two or more was tested using several in vitro bioactivity parameters. These included assays against 25 different bacteria, pharmacological studies using a smooth muscle preparation (guinea-pig ileum) and also the predicted effect on the patient as a whole on the basis of chemical analysis (using gas chromatography). The results showed an enhancement in the antibacterial effect of some essential oils when used in mixes only when these were poor antibacterial agents on their own. The action on smooth muscle was often drastically changed when using mixes due to the combination of relaxant and contractile essential oils. A stimulant effect on patients was dependent almost entirely on a high monoterpene concentration. Due to the pharmacological changes achieved by mixing different essential oils, which cannot be predicted from animal studies, there is a possible danger in the indiscriminate usage of mixes on some susceptible patients, e.g., asthmatics and also during pregnancy and childbirth.