Adverse Event vs Adverse Reaction: What Labels List
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Adverse event

Also called adverse reaction, adverse effect, AE, side effect · Reviewed

Definition: An adverse event is an undesired effect of a drug or other treatment, from mild to life-threatening. The NCI says it is also called an adverse effect or adverse reaction. FDA labeling rules draw a line between the two: a label lists adverse reactions, the events with some reason to think the drug caused them.

Event vs reaction in FDA rules

Two words, from 21 CFR 201.57(c)(7)
TermWhat it covers
Adverse eventAnything undesired seen while people used the drug, whether or not the drug caused it
Adverse reactionAn undesirable effect reasonably associated with use of the drug, where there is some basis to believe the drug caused it

The rule says the adverse reactions section lists reactions seen in clinical trials with rates for the drug and the comparison, such as placebo, and lists reactions from spontaneous reports after approval separately; see postmarketing experience.

How the list is ordered

Within a group, the rule says reactions are listed from most to least frequent. If the frequency cannot be reliably worked out, they are listed from most to least severe. That is why some label lists read like a ranking and others do not.

Why it matters for your log

Something that happens while you take a drug is an event; whether the drug caused it is a separate question. Logging the date, time since your last dose and what you noticed gives your prescriber what they need to judge that.

Read more on Dose Track: Side effects from FDA labels · Ozempic side effects · Mounjaro side effects

General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.
Postmarketing experiencePostmarketing experience is the part of a label, usually section 6.2, that lists side effects reported… PlaceboA placebo is an inactive substance or other treatment that looks the same as, and is given the same way… Injection site reactionAn injection site reaction is any change at the spot where a shot went in: redness, swelling, itching,… AnaphylaxisAnaphylaxis is a severe, whole-body allergic reaction that can be life-threatening. MedlinePlus says it…

Frequently asked questions

Is an adverse event the same as a side effect?+
Not always. An adverse event is anything undesired seen during treatment. FDA labeling rules list adverse reactions, which are the events with some basis to believe the drug caused them.
Why are some label side effects listed without percentages?+
FDA rules say that if frequency cannot be reliably determined, reactions are listed by severity, and reactions from spontaneous reports after approval are listed separately from trial results.

Sources

  1. NCI Dictionary of Cancer Terms: adverse event
  2. 21 CFR 201.57: content of prescription drug labeling (adverse reactions, paragraph (c)(7)), Cornell LII

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