Drug safety challenges have bedeviled the US Food and Drug Administration (FDA) for many years, and for good reason. The phased approach to drug development is primarily geared to establishing evidence of efficacy. Hundreds, perhaps thousands, of patients are exposed to an investigational drug in clinical trials. However, it may not be until after approval, when the medication is used by as many as hundreds of thousands of patients, that rare but serious adverse events appear or there is sufficient statistical power to determine whether the drug increases rates of more common potential adverse outcomes, such as cardiovascular disease or diabetes.
Sharfstein JM, Kesselheim AS. The Safety of Prescription Drugs. JAMA. 2015;314(3):233-234. doi:10.1001/jama.2015.7151