Adverse drug reactions - allergy? side-effect? intolerance?

BACKGROUND Adverse drug reactions (ADRs) vary from life-threatening anaphylaxis to minor common side-effects. OBJECTIVE To provide an overview on the assessment of ADRs. To discuss the features of what may be described as a 'reaction to a drug' in order to highlight those suggestive of allergy, side-effect or intolerance, and what implications this might have for the future use or avoidance of the drug. DISCUSSION Assessment of an ADR may apply to a current reaction or a history of a past reaction. The main decision is whether to cease the drug and/or whether it can be used again. Some ADRs are serious and likely to be reproducible and constitute absolute contraindications, whereas others are mild and may or may not occur on subsequent exposure. The mechanism of the ADR may be helpful in risk assessment. Drug allergy has immunological mechanisms: it may be severe, tends to be reproducible and may cross-react with structurally related drugs. Drug side-effects are more common and predictable, vary in severity and depend on the drug's pharmacological action. Intolerance tends to be less severe, and may depend on susceptibility factors, which can vary. The decision to prescribe a drug where there is a history of a previous ADR requires careful assessment of the risks and potential benefits.