What is the difference between an 'adverse event' and a 'side effect'?
Updated: 2026-09-09 · Safety and evidence
Short answer
An adverse event is any health problem occurring during a study or after using a product, whether or not it was caused by it. A side effect implies the product is held responsible. That is why pharmacovigilance reports talk about events: they record what happened, not what was proven.
In plain English
- Reporting systems are voluntary and do not verify causality for each report.
- An event appearing in a database does not prove the product caused it.
- Causality is assessed against criteria: timing, plausibility, recurrence on re-exposure, comparative rates.
- Approved labels distinguish attributed adverse reactions from observed events.
What we know
- The FDA states explicitly that FAERS reports do not by themselves establish causation.
- In trials, adverse events are collected in every arm, including placebo.
What we don't know yet
- Under-reporting makes true frequencies of uncommon events hard to estimate.
- For unapproved products there is no equivalent surveillance system.
Conceptual information about how safety reports are classified. Any symptom that worries you belongs to a healthcare professional.
Sources
External links to official sources or peer-reviewed studies.