An adverse event encompasses any unfavorable and unintended sign, symptom, or disease temporally associated with the use of an investigational product or study procedure, whether or not considered related to the treatment. This deliberately broad definition ensures comprehensive safety monitoring by capturing all medical occurrences during a trial, allowing patterns and potential safety signals to emerge that might not be apparent if only suspected drug-related events were recorded. The definition explicitly states that causality need not be established; the temporal association alone is sufficient to trigger documentation requirements.

Adverse events range in severity from minor inconveniences, such as mild headaches or temporary nausea, to life-threatening conditions requiring immediate medical intervention. Severity is typically graded using standardized scales such as mild, moderate, or severe, or using the Common Terminology Criteria for Adverse Events which provides detailed grading criteria for specific event types. Clinical trials establish systematic processes for identifying, documenting, and evaluating adverse events throughout the study, with clear procedures for reporting events to the sponsor and, where required, to regulatory authorities.

The distinction between adverse events and adverse drug reactions is clinically and regulatorily significant. While an adverse event is any untoward occurrence regardless of suspected cause, an adverse drug reaction implies a reasonable possibility of causal relationship with the investigational product. This distinction influences reporting requirements and timelines, with suspected adverse drug reactions often triggering more stringent notification requirements, particularly when they are both serious and unexpected. Site personnel must be trained to recognize and appropriately document all adverse events while also providing causality assessments for serious events.

Comprehensive adverse event collection and analysis represents a cornerstone of clinical trial safety evaluation. Investigators must diligently identify and record all adverse events occurring during the study period, even those that seem unrelated to treatment. These data enable safety reviewers and regulatory authorities to identify patterns that might indicate previously unrecognized risks. The aggregate adverse event profile from clinical trials ultimately informs the product's safety labeling and guides prescribers and patients in making treatment decisions.