Drug interaction occurs when two or more drugs, taken together, cause a (usually undesirable) combined effect. In some cases, these interactions may be lethal. It is therefore very important that both physician and patient be aware of all the medications taken concurrently; this includes prescription medications as well as over-the-counter drugs, vitamins and other dietary supplements. For example, the herbal supplement ginkgo biloba, marketed as memory-booster, is generally considered to be safe at low doses. However, ginkgo can interact with anticoagulant drugs (including aspirin) to cause spontaneous bleeding, and it can interact with some diuretics to cause high blood pressure.

The possibility of adverse drug interactions increases when patients take several medications or supplements each day. In recent years, data from the National Institutes of Health have consistently ranked adverse drug interactions as one of the leading causes of death in the US.