Adverse Drug Reactions & Pharmacovigilance: A Detailed Guide

An adverse drug reaction (ADR) is any noxious, unintended response to a drug at doses used for prophylaxis, diagnosis, or therapy. Classifying ADRs by mechanism makes them predictable — and is one of the most reliably examined topics in pharmacology.

First, three terms students mix up:

• Side effect — an unintended effect at therapeutic dose (often Type A).
• ADR — a harmful, unintended response to a drug.
• Adverse drug event (ADE) — any harm during treatment, which may or may not be caused by the drug.

Type A (Augmented) — the common, predictable ones

An exaggeration of the drug’s known pharmacological action. Dose-related, predictable, common, low mortality, usually managed by dose reduction. Examples: bradycardia with β-blockers, hypoglycaemia with insulin, bleeding with anticoagulants, sedation with benzodiazepines.

Type B (Bizarre) — the dangerous, unpredictable ones

Not related to dose or known pharmacology; rarer but higher mortality. Sub-mechanisms worth knowing:

• Immunological (drug allergy) — Gell & Coombs Types I–IV (e.g. Type I anaphylaxis with penicillin; Type IV contact dermatitis).
• Idiosyncratic / pharmacogenetic — e.g. haemolysis with oxidant drugs in G6PD deficiency; prolonged apnoea with suxamethonium in pseudocholinesterase deficiency.
• Pseudoallergic — direct mast-cell activation without prior sensitisation (e.g. vancomycin “red man syndrome”).

Types C–F at a glance

• C (Chronic): related to cumulative dose/duration — steroid-induced osteoporosis, analgesic nephropathy.
• D (Delayed): teratogenesis, carcinogenesis, tardive dyskinesia.
• E (End-of-use): rebound on withdrawal — clonidine, β-blockers, opioids, benzodiazepines.
• F (Failure): therapeutic failure — OCP failure with enzyme inducers, antibiotic resistance.

Severe reactions you must recognise

• Anaphylaxis — Type I; treat with IM adrenaline.
• SJS / TEN — severe cutaneous reactions (carbamazepine, allopurinol, sulfonamides).
• DRESS — drug reaction with eosinophilia and systemic symptoms.

Pharmacovigilance

The science of detecting, assessing, and preventing ADRs once a drug is marketed.

• Spontaneous reporting: PvPI (India), Yellow Card (UK), MedWatch (US) — anyone may report.
• Causality assessment: the WHO–UMC categories (certain → unlikely) and the Naranjo algorithm.
• Signal detection → label changes, restrictions, or withdrawal.

Exam tip: Type A = Augmented — predictable, dose-related, common (“too much of the expected effect”). Type B = Bizarre — unpredictable, immunological, rare but dangerous. The other four follow: Chronic, Delayed, End-of-use, Failure.

Reporting suspected ADRs is how unsafe drugs get flagged and withdrawn — every report strengthens the safety of the entire system.