The sponsoring company is required to constantly monitor its medication for both safety and efficacy for as long as it is being used. Since drug testing usually only involves several thousand test patients, rare side effects may not be seen during the drug trials. The sponsor is responsible for monitoring these rare side effects and notifying the FDA of their occurrence.

Occasionally rare but very serious adverse effects come to the notice of the pharmaceutical company. Sometimes, in the opinion of the FDA, these rare but serious effects do not mandate removal of the product from the market but do require notification to the prescribers. To insure that these potentially severe untoward effects are brought to the attention of the prescribers the FDA mandates a warning be published in the PI Sheet. To emphasize the concern, this warning is separated from the other warnings, cautions and alerts and surrounded in the PI sheet by a Black Box border around the text. This is designed to catch the attention of all concerned: prescribers; pharmacists; and the public. The FDA makes this warning because the drug carries a significant risk of serious or possibly even life-threatening adverse effects. Because the warning is boxed off from the rest by of the PI Sheet by a black ink border it is called a “Black Box Warning” or a “Black Label Warning.”