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A drug for a rare disease is evaluated in a small clinical trial. The treatment is modestly effective with manageable side effects. Balanced against the devastating nature of the disease, the drug is benefiting patients more than it is causing harm. Regulators approve it.

Months later, physicians prescribing the drug widely start to note a small number of patients are dying, despite the fact that no deaths were reported in a clinical trial of the drug. Similarly, the frequency of serious side effects rises dramatically, including some so severe that patients require hospitalization. The drug’s real-world safety profile appears worse than what was characterized in the small clinical trial.