A black box warning is the strongest form of warning issued by the FDA about a drug, the step taken just short of removing the drug from the market. It's a recognition of how harmful the drug can be if given to patients who are at risk of developing the possible adverse side effects.
In July 2008, the FDA began to require a class of drugs called flouroquinolones to carry a black box warning. Flouroquinolones, with brand names like Cipro and Levaquin, are strong antibiotics, often used to clear up infections like urinary tract infections or pneumonia. A small percentage of patients found themselves with tendon ruptures from the drugs.
In November 2007, a black box warning was added to Avandia, a drug developed for patients with diabetes. Many diabetes patients also suffer from heart disease, and Avandia was found to increase their chances of suffering heart attacks.