Cross-Reactivity and Drug Rechallenge in Idiosyncratic Drug-Induced Severe Neutropenia and Agranulocytosis

Idiosyncratic drug-induced severe neutropenia and agranulocytosis are rare but potentially life-threatening adverse drug reactions. They cause profound neutrophil depletion after exposure to non-cytotoxic medications. Advances in supportive care have substantially reduced mortality. Long-term management nevertheless remains challenging. Clinicians must decide whether a structurally or pharmacologically related drug can be prescribed safely (cross-reactivity). They must also decide whether the culprit drug can ever be reintroduced (rechallenge). Current evidence supports an immune-mediated pathogenesis. It involves reactive drug metabolites, hapten formation, human leukocyte antigen (HLA)-restricted antigen presentation, and T-cell activation. The strength of evidence for cross-reactivity varies considerably across drug classes. Clinically meaningful cross-reactivity is well established for antithyroid thionamides. Evidence for beta-lactam antibiotics and thienopyridine antiplatelet agents remains limited, resting largely on case reports and pharmacovigilance data. For most other medications, evidence is insufficient to support generalized class-wide avoidance. Evidence supporting drug rechallenge is similarly heterogeneous. The most robust data concern clozapine. Carefully selected patients with previous neutropenia—but generally not agranulocytosis—may successfully undergo rechallenge under intensive hematologic monitoring, often with granulocyte colony-stimulating factor (G-CSF) support. Outside this setting, deliberate rechallenge is rarely justified. It should be reserved for exceptional clinical circumstances when no effective therapeutic alternative exists. This review summarizes current mechanistic insights. It evaluates the available evidence for cross-reactivity and rechallenge across major drug classes, discusses the emerging role of pharmacogenomics, and proposes a practical framework to guide individualized clinical decision-making. Permanent discontinuation of the culprit drug remains the standard approach. Advances in immunology, pharmacogenomics, and international pharmacovigilance registries are progressively enabling more personalized risk assessment.