Drug Reactions

Drug reactions can be classified in many ways. One useful approach is to separate predictable reactions occurring in normal patients from unpredictable reactions occurring in susceptible patients.

Predictable adverse reactions;

• Over dosage (wrong dosage or defect in drug metabolism)
• Side effects (sleepiness from antihistamines)
• Indirect effects (antibiotics changing normal flora)
• Drug interactions (altered metabolism of drugs; most commonly involving the cytochrome P-450 enzymes)

Unpredictable adverse reactions

• Allergic reaction (drug allergy or hypersensitivity; immunologic reaction to drug; requires previous exposure or cross-reaction)
• Pseudo allergic reaction (non-immunologic activation of mast cells).
• Idiosyncratic reaction (unexplained reaction, not related to mechanism of action, without known or suspected immunologic mechanism).

Note: 80% of allergic and pseudo allergic drug reactions are caused by Beta-lactam antibiotics, aspirin, NSAIDs, and sulfonamides