Clinical trials enroll a few thousand people. Once a drug reaches the market, millions take it, often alongside other medications and with conditions the trials excluded. Pharmacovigilance is the science of monitoring the safety of drugs after they are approved. It collects reports of adverse events, analyzes them for patterns, and takes action when new risks emerge. The work is ongoing, and it can change a drug's label, restrict its use, or remove it from the market entirely.

Systems vary by country but share common features. Health professionals and patients can report suspected adverse reactions to national authorities. Manufacturers must forward serious reports they receive within tight deadlines. Regulators maintain databases, such as the FDA's FAERS and the EU's EudraVigilance, and use statistical methods to detect signals. A signal is a possible causal relationship between a drug and an adverse event that warrants further investigation. Signals can come from spontaneous reports, published literature, or observational studies. When a signal is confirmed, regulators may update the label, issue a safety communication, or require a risk management plan.

Key activities:

• Spontaneous reporting. Health professionals and patients report suspected reactions.
• Signal detection. Statistical analysis of databases to find patterns.
• Risk assessment. Evaluating the strength of evidence for a causal link.
• Risk management. Label changes, restrictions, or withdrawal.
• Periodic safety reports. Manufacturers submit regular updates.

Pharmacovigilance depends on reporting. Underreporting is a persistent challenge, which is why regulators encourage both clinicians and patients to submit reports.