Pharmacovigilance, Adverse Event Reporting, and Post-Market Surveillance

Pharmacovigilance is the science and activities relating to the detection, assessment, understanding, and prevention of adverse effects and other drug-related problems. Because pre-marketing trials enrol limited numbers of patients for limited periods, rare or delayed harms often emerge only once a medicine is in widespread use, which is why post-market surveillance and adverse event reporting are essential to drug safety.

Definition

Pharmacovigilance is the set of scientific activities concerned with detecting, assessing, understanding, and preventing adverse effects and other problems related to medicines, typically operating through systems that collect reports of suspected adverse reactions from health professionals, patients, and manufacturers after a product reaches the market.

Scope

This entry covers the rationale for surveillance after marketing, spontaneous reporting systems and their role in signal detection, the chronic problem of under-reporting, and the distinction between detecting a signal and confirming a causal hazard. It is a reference description of how safety surveillance works; it does not give clinical guidance on any specific medicine.

Key concepts

• Post-marketing surveillance
• Spontaneous reporting systems
• Signal detection
• Under-reporting
• Disproportionality analysis
• Individual case safety report
• WHO international drug monitoring programme
• Benefit-risk reassessment

Mechanisms

Pharmacovigilance largely rests on spontaneous reporting, in which clinicians, pharmacists, patients, and companies submit reports of suspected adverse reactions to national centres and a shared international database. Aggregating these individual case safety reports allows analysts to detect signals, that is, patterns suggesting a previously unrecognised or insufficiently documented hazard, often using disproportionality methods that flag drug-event pairs reported more often than expected. A signal is a hypothesis, not proof; it triggers further evaluation through additional studies before regulatory action. The system's principal limitation is under-reporting, since only a fraction of reactions are ever reported, which means spontaneous data can detect signals but cannot reliably estimate incidence.

Clinical relevance

Reporting suspected reactions and interpreting safety communications are part of professional practice, and understanding the strengths and limits of surveillance data informs how safety information should be weighed. This entry describes the surveillance system as a matter of reference understanding and is not a basis for individual diagnostic or treatment decisions.

Epidemiology

Adverse reactions impose a measurable burden, with prospective studies attributing a meaningful share of hospital admissions to them, yet spontaneous reporting captures only a minority of events. A systematic review found that the median under-reporting rate across studies was high, underscoring that surveillance databases are suited to signal detection rather than to estimating how frequently reactions occur.

History

Modern pharmacovigilance was founded in response to the thalidomide tragedy of the early 1960s, after which the World Health Organization established an international drug-monitoring programme and member states created national centres to collect reports of suspected reactions. The discipline subsequently developed formal methods for signal detection and broadened from passive spontaneous reporting toward more active surveillance approaches.

Debates

How much does under-reporting limit spontaneous systems?

Because only a small and variable fraction of reactions are reported, spontaneous systems are well suited to detecting signals of new hazards but cannot measure incidence, and reliance on them risks both missed and spuriously prominent signals.

Key figures

• I. Ralph Edwards
• Jeffrey K. Aronson
• Lorna Hazell
• Saad Shakir

Seminal works

• edwards-aronson-2000
• hazell-shakir-2006

Frequently asked questions

Why is surveillance needed after a drug is approved?

Pre-marketing trials involve relatively few patients followed for limited time, so rare, delayed, or population-specific harms often only become apparent once a medicine is used widely, which post-market surveillance is designed to capture.

What is a safety signal?

A signal is reported information suggesting a possible new association between a drug and an adverse event; it is a hypothesis to be investigated further, not confirmed proof that the drug causes the harm.