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Causality assessment · Causality: well established

Clozapine and Agranulocytosis / severe neutropenia: is the FAERS signal real?

Agranulocytosis: n=2,074 · PRR 21.1 · ROR 21.5
Clozapine side-effect profileAll assessmentsSee in the full list
A written Bradford Hill assessment of one drug–event pair. The grade is a judgement about the signal, not a statement that this drug causes this event in any one person; FAERS counts are reports, not rates.
Causality: well established

Clozapine ↔ Agranulocytosis / severe neutropenia

Agranulocytosis: n=2,074 · PRR 21.1 · ROR 21.5 Neutropenia: n=18,898 · PRR 27.1 · ROR 31.8

Reporting over time (stimulated-reporting check)

2004: 41 reports (2.40% of the drug's reports)2005: 37 reports (2.33% of the drug's reports)2006: 68 reports (2.78% of the drug's reports)2007: 24 reports (2.60% of the drug's reports)2008: 23 reports (2.47% of the drug's reports)2009: 33 reports (1.65% of the drug's reports)2010: 38 reports (1.32% of the drug's reports)2011: 99 reports (1.76% of the drug's reports)2012: 102 reports (2.44% of the drug's reports)2013: 41 reports (1.36% of the drug's reports)2014: 42 reports (1.24% of the drug's reports)2015: 68 reports (1.63% of the drug's reports)2016: 137 reports (3.02% of the drug's reports)2017: 148 reports (2.03% of the drug's reports)2018: 201 reports (2.95% of the drug's reports)2019: 179 reports (2.40% of the drug's reports)2020: 66 reports (0.82% of the drug's reports)2021: 75 reports (1.02% of the drug's reports)2022: 67 reports (0.48% of the drug's reports)2023: 339 reports (2.58% of the drug's reports)2024: 85 reports (0.73% of the drug's reports)2025: 134 reports (1.62% of the drug's reports)05081114172023

No reporting spike: the pair's share of the drug's reports is fairly stable over time (busiest two years hold 21% of 2,047 reports), so stimulated reporting is unlikely to explain the signal.

Bars: reports per year as a share of all FAERS reports for this drug, 2004–2025; amber = spike year (>3× the median share, ≥50 reports). Source: openFDA count=receivedate, retrieved 2026-10-09.

Clozapine-induced agranulocytosis is arguably the most consequential single adverse-drug-reaction discovery in modern psychiatry: it is the reason clozapine, despite being one of the most effective antipsychotics for treatment-resistant schizophrenia, can only be dispensed in the U.S. through a mandatory REMS program requiring regular absolute neutrophil count (ANC) monitoring for the life of the prescription — weekly for the first 6 months, biweekly for the next 6, then monthly if counts stay in range. Reported incidence is 1–2% of patients, overwhelmingly concentrated in the first 6 months of therapy, which is itself a form of temporal and dose-independent specificity (the event is a sudden, often zero-count crash, not a gradual dose-related decline). The mechanism remains formally unresolved — proposed as an idiosyncratic, likely immune-mediated toxic effect on bone marrow myeloid precursors or their progenitors — but the monitoring program's mandatory, indefinite existence is itself regulators' institutional verdict that the causal link is settled, not merely a statistical association.

Regulatory basis: Clozapine REMS program (mandatory ANC monitoring); clozapine prescribing information.

Bradford Hill viewpointStatusNote
Temporality (necessary)metRisk overwhelmingly concentrated in the first 6 months — a well-characterized window
StrengthmetPRR 21–27
ConsistencymetReplicated across countries and registries for decades
SpecificitymetRisk is unusually high for clozapine among antipsychotics specifically
Biological gradientpartialIdiosyncratic reaction; not clearly dose-dependent the way the TD example above is
PlausibilitymetProposed immune-mediated bone-marrow toxicity mechanism
CoherencemetConsistent with a myeloid-toxicity pattern
ExperimentmetMandatory ANC monitoring (REMS) demonstrably reduces fatal outcomes — a real-world intervention
AnalogypartialOther drugs (older antipsychotics, antithyroid agents) also cause agranulocytosis, but at lower rates

This page's own qualitative read, not an automated/citation-gated assessment.

✗ No ClinicalTrials.gov results-posting trial in the top 15 search hits for clozapine captured agranulocytosis or neutropenia at a usable rate. Not surprising: this is exactly the signal the mandatory ANC-monitoring REMS program exists to catch in ongoing real-world use, not something most individual registered trials are sized or designed to surface in their own AE tables.