Drugs that have special problems, particularly ones that may lead to death or serious injury, may have this warning information displayed within a box in the prescribing information. This is often referred to as a "boxed" or "black box" warning. Drugs that have such boxed warnings are not permitted to have reminder ads.
Source: FDA Glossary; Drug Advertising - A Glossary of Terms
A boxed warning, commonly referred to as a "black box" warning, is the most serious type of warning mandated by the U.S. Food and Drug Administration (FDA). They are prominently featured in the labeling of drugs to warn prescribers about serious adverse reactions or special problems. Boxed warnings are displayed on a drug's package insert, in the Physicians' Desk Reference, on the FDA's Web site, and on the Web sites of drug marketing companies. Physicians are required to provide patients with information about relevant risks, but they can use professional judgment to decide whether to prescribe a drug with a boxed warning.
Source: American Family Physician, FDA Boxed Warnings: How to Prescribe Drugs Safely.
Economists and physicians have thoroughly studied the effects of FDA boxed warnings on prescription patterns. It is not necessarily true that a physician and patient will have a conversation about a drug's boxed warning after it is issued. For instance, an FDA-mandated boxed warning decreased rosiglitazone use by 70%, but that still meant 3.8 million people were given the drug. Later research indicated that after receiving an FDA advisory, there was a decrease in rosiglitazone use, due to a combined effect of media exposure, advisory, and scientific publications, whereas pioglitazone (with a similar advisory) did not similarly decrease in use. This challenges the idea that physicians and patients will have a conversation after a boxed warning is issued. Throughout the country, boxed warnings will be translated into prescription patterns differently by different physicians.
Source: Wikipedia, Boxed warning.