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How do regulators monitor harmful reactions after drugs reach the market?

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A lab technician wearing protective gloves and face shield lifts a rack of deep-frozen cells from a cryopreservation tank emitting vapor in a laboratory setting. — pharmaceutical industry A technician withdrawing deep-frozen cells for in vitro culture at a laboratory of a German pharmaceutical company.

Because clinical trials involve a limited number of participants, some rare or unforeseen adverse reactions may not be identified until a drug is widely available to the general population. To monitor these risks, regulatory agencies like the Food and Drug Administration (FDA) rely on postmarketing surveillance.1

Pharmaceutical companies are required to report adverse drug events to the FDA on a regularly scheduled basis. Identifying these events is complex because symptoms like fever or hepatitis can occur independently of medication. To determine a cause-and-effect relationship, experts may observe if an adverse event resolves after stopping the drug or reappears upon re-administration, though re-exposure is often avoided for serious reactions.1

This process is ethically challenging; while regulators must protect the public from harm, they must also avoid premature warnings that could deprive severely ill patients of necessary treatments. Consequently, there have been instances where manufacturers and regulators faced criticism for delays in identifying or communicating risks when data was insufficient to establish a clear link.1