There is a general belief that herbal products, thus essential oils too, are safe because they are natural. Together with the increased appearance of common and self-initiative usage of herbal preparations there is also an increased need of knowing and understanding possible interactions with concomitantly taken drugs. Since essential oils are mixtures of multiple active volatile compounds it is very likely for essential oils and especially their constituents also to interact with drugs. The presented treatise will reveal some of such interactions. Most of them are pharmacokinetic ones, while pharmacodynamic interactions seem to be less studied. While some of these interactions may have a negligible impact on the effect of drugs--and it was the aim of this paper to demonstrate the impact of volatile oils on the effect of drugs when applied together--other mutual effects may be rather harmful. Under certain circumstances it could be possible to use the effects of these interactions also for the benefit of the patients, but further clinical investigations are needed.