Monoterpenes are found in the volatile essence of flowers, plants oils, and herbal medicines. Some are commonly used as food additives and fragrance components, and many are found in cosmetics, soaps, cleaning products, disinfectants, preservatives, and medicines. We have recently discovered a moderate inhibitory effect of borneol and isoborneol toward CYP2B6-catalyzed bupropion hydroxylase activity. Based on that result, we expanded our study to evaluate the inhibitory effects of 22 monoterpenoids on CYP2B6 activity in vitro. Among the monoterpenoids screened, borneol, camphor, cineole, isoborneol, menthol, and perillaldehyde showed slight inhibition of CYP2B6-catalyzed bupropion hydroxylation, displaying greater than 50% inhibition at 50muM. Citral and geraniol strongly inhibited CYP2B6 hydroxylase activity in a competitive manner, with K(i) values of 6.8 and 10.3muM, respectively, which are higher than the K(i) (1.8muM) of the well-known CYP2B6-selective inhibitor thio-TEPA. These in vitro data indicate that high amounts of these two monoterpenoids might interact with drugs that are metabolized by CYP2B6. The in vivo pharmacokinetics of these compounds should be examined to determine whether the inhibition of CYP2B6 activity by monoterpenoids has clinical relevance.