# Side effect versus adverse event

Source: https://onco.cc/terms/side-effect-vs-adverse-event/  
OnCo record `side-effect-vs-adverse-event` (Term). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

An adverse event is anything bad that happens to a patient during a trial, whether or not the drug caused it; a side effect is a harm the drug is known to cause. Trial tables list adverse events and then judge which were treatment-related.

## Summary

Counting every adverse event, including those from the cancer itself or from unrelated illness, is deliberate: it avoids under-reporting and lets the two arms be compared fairly, since the control arm has adverse events too. Investigators then attribute each event as related or unrelated to treatment, yielding 'treatment-related adverse events', and grade its severity; drug labels list the side effects seen more often on the drug than on placebo. Some harms appear only years later (heart damage from anthracyclines, second cancers after radiation, permanent hormone deficiencies after immunotherapy) and are called late effects, and survivorship care exists to watch for them.

## Fields

- Kind: Term
- Last checked: 2026-09-09
- Also known as: side effect; side effects; side-effect; side-effects; adverse event; adverse events; AE; AEs; adverse effect; adverse effects; adverse reaction; adverse reactions; any-grade; all-grade; grade 1-2; low-grade toxicity; late effects; long-term effects; late toxicity; cumulative toxicity

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Adverse_event
- Wikipedia: https://en.wikipedia.org/wiki/Adverse_event

## Connected records

- terms: [Dose](https://onco.cc/terms/dose/), [Immune-related adverse events (irAEs)](https://onco.cc/terms/irae/), [Placebo](https://onco.cc/terms/placebo/), [Quality of life](https://onco.cc/terms/quality-of-life/), [Toxicity grade](https://onco.cc/terms/toxicity-grade/)
- technologies: [Cardio-oncology](https://onco.cc/technologies/cardio-oncology/), [Survivorship care and late-effects surveillance](https://onco.cc/technologies/survivorship-care-plan/)

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