Table 1:

Classification of unpredictable adverse drug reactions1,8

IntoleranceOccurs at very low doses and is not due to underlying abnormalities of metabolism, bioavailability or excretionASA-induced tinnitus
IdiosyncracyUnrelated to intended pharmacologic action of the drug, is reproducible, and is usually due to abnormalities of metabolism, excretion or bioavailabilityPrimaquine-induced hemolytic anemia in glucose-6-phosphate dehydrogenase deficiency
AllergyImmune mediated:
Type 1 (IgE-mediated)
Type 2 (cytotoxic)
Type 3 (immune complex)
Type 4 (delayed hypersensitivity; subclassified into monocytes (IVa), eosinophils (IVb), CD8+ T lymphocytes (IVc), neutrophils (IVd)
Type 1: anaphylaxis
Type 2: thrombocytopenia, anemia
Type 3: serum sickness, vasculitis
Type 4: benign drug exanthems, DRESS, AGEP, contact dermatitis
“Pseudoallergy”Similar to type I allergic reactions with different pathophysiology (direct mediator release, often histamine, from mast cells and basophils)Opiate-induced urticaria
  • Note: AGEP = acute generalized exanthematous pustulosis, ASA = acetylsalicylic acid, DRESS = drug reaction with eosinophilia and systemic symptoms, IgE = immunoglobulin E.