The problem of herb-drug interactions is being increasingly recognised. However, except for hypericum (Hypericum perforatum L.; St John’s wort), data on drug interactions with other herbs are still inconclusive, although research has started to fill in the gaps, and knowledge is being gained about this important safety issue. This review summarises clinical data on interactions between drugs and herbal medicines, which were compiled using the following databases: MEDLINE via PubMed, Web of Science® (from their inception until March 2005) and the Cochrane Library (until 2005, issue 1). Clinical data on herb-drug interactions, observed in case reports, case series or clinical trials, were included. Results are described for the following herbs: Devil’s claw (Harpagophytum procumbens D.C.), echinacea, evening primrose (Oenothera biennis L.), garlic (Allium sativum L.), ginkgo (Ginkgo biloba L.), goldenseal (Hydrastis canadensis L.), hawthorn, kava (Piper methysticum Forst.), milk thistle (Silybum marianum [L.] Gaertn.), peppermint, saw palmetto (Serenoa repens [Bartel] Small) and valerian. Information on herb-drug interactions is discussed through use of in vivo and experimental data, and conclusions are drawn when possible.