The consumption of herbal medicines is increasing steadily throughout the world, although to our knowledge there are neither studies on their effectiveness nor controls over the quality and safety of these preparations. Considered "food integrators", these preparations are marketed without restriction. It is a common notion that natural therapy has neither side nor toxic effects: allergic reactions, direct toxic effects or those due to contamination, carcinogenicity, mutagenicity, and heavy metal toxicity have been reported as adverse events caused by herbs. Rather than replacing traditional therapy, most herbal medical treatment is used in conjunction with it. Also, the attending physician is generally not informed that the patient is using herbs. Because Passionflower, hydroalcoholic extracts, Juniper and Verbena officinalis supply variable quantities of vitamin K, they can lessen the effect of oral anticoagulant therapy. Ganoderma Japonicum, Papaw, Salvia miltiorrhiza, Ginseng, Devil's claw, Garlic, Quinine, Ginkgo, Ginger, Red Clover and Horse-Chestnut reinforce warfarin action by heterogeneous mechanisms. They should thus not be used in patients on oral anticoagulant and/or antiplatelet therapy. The scientific community must take into account the adverse events caused by interaction between herbal medicine and conventional therapy, and patients must be informed of the dangers of these preparations. If a bleeding event occurs or the quality of anticoagulant therapy is poor, the clinician should consider the possibility of interaction between conventional therapy and herbal medicine that the patient has neglected to mention he is taking.