In recent years, the issue of herbal medicine-drug interactions has generated significant concern. Such interactions can increase the risk for an individual patient, especially with regard to drugs with a narrow therapeutic index (e.g. warfarin, ciclosporin and digoxin). The present article summarizes herbal medicine-drug interactions involving mainly inhibition or induction of cytochrome P450 enzymes and/or drug transporters. An increasing number of in vitro and animal studies, case reports and clinical trials evaluating such interactions have been reported, and the majority of the interactions may be difficult to predict. Potential pharmacodynamic and/or pharmacokinetic interactions of commonly used herbal medicines (black cohosh, garlic, Ginkgo, goldenseal, kava, milk thistle, Panax ginseng, Panax quinquefolius, saw palmetto and St John's wort) with conventional drugs are presented, and sometimes the results are contradictory. Clinical implications of herbal medicine-drug interactions depend on a variety of factors, such as the co-administered drugs, the patient characteristics, the origin of the herbal medicines, the composition of their constituents and the applied dosage regimens. To optimize the use of herbal medicines, further controlled studies are urgently needed to explore their potential for interactions with conventional drugs and to delineate the underlying mechanisms.