Adverse reactions to drugs are common. Only some are true allergic reactions. An adverse drug reaction is any response to a drug that is unwanted and unintended.
In many countries they are a frequent cause of hospitalisation and mortality. Sometimes the reactions are known side-effects of the drug, but some may be unrecognized.
Adverse drug reactions occur at the usual, or intended dose of the drug. Examples include:
- A predictable side-effect from the action of the drug, just more than usually seen (e.g. getting more drowsy than most people after taking powerful painkillers).
- A rare, unexpected reaction that isn’t part of the drug’s usual mechanism (e.g. allergy or anaphylaxis).
- A chronic reaction only seen after prolonged exposure to a drug.
- A delayed reaction that occurs some time after a drug has been ceased.
- Reactions due to withdrawal that happen immediately when certain drugs are ceased.