In simple terms
A side effect is an unwanted result of taking a medicine. The US Food and Drug Administration describes side effects as unwanted undesirable effects that are possibly related to a drug.
They are not a sign that something has gone wrong with the medicine. Almost any drug that does something useful can also do something unwanted.
How it works
A drug rarely acts in only one place. It travels through the body and reaches tissues beyond its target, which is where most familiar side effects come from.
Who gets them varies. FDA notes that age, other medicines, vitamins, dietary supplements and existing health conditions all affect whether a side effect appears.
Regulators treat a side effect as serious when it results in death, is life threatening, or causes hospitalization, disability or permanent damage, or a birth defect after exposure before conception or during pregnancy.
The wording differs between regulators. The European Medicines Agency defines an adverse event as an untoward medical occurrence after exposure to a medicine, which is not necessarily caused by that medicine. A side effect implies the medicine probably was the cause. An adverse event does not.
Why it matters
Side effects are half of every decision about a medicine, weighed against the expected benefit. That balance looks different for a short course of antibiotics than for a drug taken for decades.
Many only appear once a drug is used widely, so reporting systems keep collecting them after approval. In the United States that system is MedWatch.
Where you’ll see it
- The leaflet inside a medicine package
- Drug safety alerts and label changes
- National databases of suspected adverse reactions
- Clinical trial results reporting harms as well as benefits
Example
An antibiotic prescribed for a chest infection clears the infection and also upsets the stomach. The stomach upset is the side effect.
Often confused with
A side effect is not an allergy. An allergic reaction is the immune system responding to the drug, and it can follow a dose that caused no trouble before.
Key facts
- FDA describes side effects, also called adverse reactions, as unwanted undesirable effects that are possibly related to a drug.1
- Both prescription and over the counter medicines can cause side effects.1
- A side effect is considered serious if it results in death, is life threatening, or causes hospitalization, disability or permanent damage, or a birth defect after exposure before conception or during pregnancy.1
- Age, other medicines, vitamins, dietary supplements and existing health conditions all affect who experiences side effects.1
- The European Medicines Agency defines an adverse event as an untoward medical occurrence after exposure to a medicine, which is not necessarily caused by that medicine.2
This entry explains a medical term. It is not medical advice. If you are concerned about your health, speak with a qualified health professional.
Related concepts
In the news
Quick checkHow does an adverse event differ from a side effect?Show answer
An adverse event is anything that happens after taking a medicine, whether or not the medicine caused it.
Sources
- U.S. Food and Drug Administration. Finding and Learning about Side Effects (adverse reactions). 8 August 2022 (accessed 22 September 2026)
- European Medicines Agency. Adverse event, EMA glossary. Undated (accessed 22 September 2026)
Editorially reviewed by Specialty Digest Editorial TeamLast reviewed September 22, 2026Researched and drafted with AI assistanceReport an issue