How do an adverse event and an adverse drug reaction differ?

Why

Begin with the observation rather than the conclusion. A patient receiving an antibiotic develops a headache. The headache is an adverse event because it occurred during exposure.

It becomes a suspected adverse drug reaction when a reasonable possibility of a connection is considered, including situations where spontaneous reporting conventions treat reports as suspected reactions. Review timing, alternative causes, dose changes, medical history and prior knowledge. A suspected reaction may need reporting even when causality remains uncertain.

In clinical trials, how an investigator assesses relatedness, whether the event is serious and whether it is unexpected can affect expedited reporting. Those are different axes. Also remember that spontaneous-report datasets lack reliable exposure denominators, so report counts cannot establish incidence or causation.

Regulatory definitions and workflows vary between pre-authorisation trials and post-authorisation surveillance. Cite the applicable framework rather than inventing one rule for every case.