In the United States, a black box warning (also sometimes called a black label warning) is a type of warning that appears on the package insert for prescription drugs that may cause serious adverse effects. It is so named for the black border that usually surrounds the text of the warning.
A black box warning means that medical studies indicate that the drug carries a significant risk of serious or even life-threatening adverse effects. The U.S. Food and Drug Administration (FDA) can require a pharmaceutical company to place a black box warning on the labeling of a prescription drug, or in literature describing it. It is the strongest warning that the FDA requires.
Black box warnings on drugs received increased media attention in the United States in 2005. Among some of the more widely covered stories:
The FDA has required that black box warnings be placed on all antidepressant medications warning they may result in increased risk of suicidal tendencies in children and adolescents.
FDA advisors have recommended that Pfizer be required to place a black box warning on their NSAID celecoxib (U.S. trade name Celebrex).
As of 17 November 2004, the FDA has required a black box warning on the Depo-Provera contraceptive injection, due to the risk of significant loss of bone density with long-term use. 
As of October 9, 2006, the FDA added a black box warning to the anticoagulant warfarin due to the risk of bleeding to death. 
In February 2006, the FDA's Drug Safety and Risk Management Advisory Committee voted to include black box warnings on methylphenidate formulations—used to treat ADHD—due to possible cardiovascular side-effects. A month later, the agency's Pediatric Advisory Committee effectively rejected recommending boxed warnings for both cardiovascular and psychiatric adverse effects. Minutes and transcripts of the relevant meetings are available on the FDA website.