Classification of ADRs

Classification of adverse drug reactions

There is no generally accepted classification for ADRs. However, one major classification can generally be made: an ADR is either related (type A) or unrelated (type B) to the pharmacological action of the drug.

The type A ADR is due to an augmented or exaggerated effect of the drug. This effect can either be derived from the primary pharmacological activity or from the secondary pharmacological activity. Usually these two types of ADR are dose-dependent, avoidable, and predictable. Examples:

  • Hypoglycaemia due to sulphonylurea derivatives (antidiabetic drugs)
  • Bradycardia due to beta blockers
  • Bleeding due to anticoagulants
  • Sedation due to GABA-ergic benzodiazepine anxiolytics
  • Sedation due to histaminergic action of monoaminergic antidepressant (dirty drug)

Type B or bizarre ADRs are not related to the primary pharmacological mechanism and involve idiosyncratic reactions and allergic reactions. These reactions are unpredictable, relatively rare, often serious, and only occur in susceptible patients. Examples:

  • Bone marrow depression due to the atypical antipsychotic clozapine
  • Anaphylaxis due to penicillin
  • Malignant hyperthermia after anaesthesia
  • Rash due to allopurinol

The causality of ADRs is often assessed using the Adverse Drug Reaction Probability Scale, also known as the Naranjo algorithm. Based on the answers to ten questions, a probability score is generated which indicates the likelihood of a causal relationship between the administration of a drug and an untoward clinical event.