Adverse event (AE)

An adverse event (AE) is any untoward medical occurrence in a patient or clinical trial participant that occurs after administration of a medicinal product but does not necessarily have a causal relationship with that product. An adverse event may consist of a sign, symptom, disease, or abnormal laboratory finding that is temporally associated with treatment.

The defining characteristic of an adverse event is the absence of a required causal relationship. Events that are considered reasonably related to a medicinal product are generally classified as adverse drug reactions (ADRs).

Regulatory definitions

ICH E6

Any unfavourable medical occurrence in a trial participant administered the investigational product. The adverse event does not necessarily have a causal relationship with the treatment 1.

Comparison with adverse drug reaction

Although the terms are sometimes used interchangeably in non-regulatory contexts, adverse event and adverse drug reaction have distinct meanings.

Adverse event

  • Any untoward medical occurrence temporally associated with administration of a medicinal product
  • Causality is not required
  • Includes events unrelated to treatment

Adverse drug reaction

  • A noxious and unintended response for which a causal relationship with the medicinal product is at least a reasonable possibility
  • Causality is suspected or established
  • Includes only events considered related to the medicinal product

See also

  • Adverse drug reaction
  • Pharmacovigilance
  • Serious adverse event
  • Suspected unexpected serious adverse reaction

References

  1. International Council for Harmonisation. 2026. ICH E6(R3): Consolidated Guideline for Good Clinical Practice.
    Available at: https://database.ich.org/sites/default/files/ICH%20E6%28R3%29_Step4_FinalConsolidatedGuideline_2026_0616_.pdf ↩︎