A black box warning is a notice on the packaging of a prescription drug which warns patients and prescribers that the drug has potentially dangerous side effects. This warning system is primarily used by the Food and Drug Administration (FDA), an American regulatory agency which oversees the safety of pharmaceuticals produced and sold in the United States. Other national regulatory agencies may use different systems to indicate that drugs are potentially dangerous.

When a drug receives a black box warning, it means that studies on the drug have suggested that it can have dangerous or even deadly side effects. Black box warnings are often added retrospectively as the result of information about a drug which has been uncovered in the course of routine usage. When doctors start reporting high levels of dangerous side effects for a prescription drug, the FDA may hold a review to decide whether or not the drug requires a black box warning.

The “black box” in “black box warning” refers to the bold black border which is drawn around the warning. The border is designed to draw attention to the warning, and to offset it from other information which may be present in pharmaceutical packaging. Thanks to widespread public awareness of the implications of a black box warning, a black box on drug packaging serves as an alert that the drug is dangerous even before people read the text of the warning.

Black box warnings must typically be included on the packaging of the drug, and in the printed inserts used to provide information about the drug to patients and prescribers. They are designed to alert patients to the potential dangerous of the drug, and they also alert pharmacists and doctors. Patients who receive a drug with a black box warning may want to discuss the warning and its implications with their doctors, as people should not take drugs with potentially deadly side effects without knowing what those side effects are and how to recognize them.

Some notable examples of black box warnings include the warnings on antidepressant drugs which indicate that these drugs can cause suicidal thoughts in teenagers and children, and the black box warning on the anticoagulant drug warfarin warning about the risk of bleeding to death while on the medication. As a general rule, drugs with a black box warning are left on the market because their benefits are viewed as more valuable than their potential dangers, but sometimes a black box drug will be removed from the market after evaluation by the FDA.