The increasing use of herbal remedies by the general public presents several challenges to healthcare professionals who may be asked to give advice on the use of such products in conjunction with other medicines. In particular, the pharmacological properties and potential interactions of herbal products are often less well understood than those of conventional medicines, increasing the risk of adverse effects and drug interactions. In addition, good quality data on herbal medicines are often lacking. Here we highlight the problem of drug-herb interactions and consider the nature of the evidence supporting drug interactions with herbal products.