After a drug has been on the market for some time, issues pertaining to its safety frequently emerge. These issues arise either through continued clinical research or more commonly, through adverse events reported by consumers. Following these concerns, the FDA has two options. It can choose to completely remove the drug from the market or it can issue an advisory, the strongest of which is a black box warning which appears on the drug's printed materials, both inside the packaging and on materials developed for the doctors who may prescribe the drug.
Black box warnings have received increased attention since 2004. At this time, the popular birth control shot, Depo-Vera was required to carry a black box warning due to the risk of significant bone density loss with long-term use. In November of 2007, the FDA added a black box warning to the diabetes medication Avandia, citing the risk of heart failure or heart attack to patients with underlying heart disease. On July 8, 2008, the FDA required manufacturers of systemic fluoroquinolone antimicrobial drugs to carry a black box warning regarding the increased risk for tendonitis and tendon rupture. Fluoroquinolone products affected by the labeling changes include ciprofloxacin (Cipro, Bayer; and generics), extended-release ciprofloxacin (Cipro XR, Bayer; Proquin XR, Depomed), gemifloxacin (Factive, Oscient), levofloxacin (Levaquin, Ortho McNeil), moxifloxacin (Avelox, Bayer), norfloxacin (Noroxin, Merck), and ofloxacin (Floxin, Ortho McNeil; and generic).
Most recently, the FDA added a black box warning to the label of the blood thinner, Plavix, after reports of some patients not being able to process it, due to a genetic variation. Plavix is a drug that is prescribed to heart disease patients to prevent dangerous blood clots. Plavix must be broken down by an enzyme, which 2 to 14% of people have low levels of, in the liver. Also known as the world's second best selling drug, it's marketed by Bristol-Myers Squibb, and had 8.6 billion in sales in 2008.
The FDA's black box warning can be found on many other drugs that have been approved as well, so it's important to understand that this label doesn't mean that you shouldn't use a particular drug, but just that you should be aware and cautious of its side effects.