Causality assessment · Causality: well established

Clozapine and Myocarditis: is the FAERS signal real?

n=2,231 co-reports · PRR 41.1 · ROR 41.8

Clozapine side-effect profile All assessments See 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 ↔ Myocarditis

n=2,231 co-reports · PRR 41.1 · ROR 41.8

Clozapine, the most effective antipsychotic for treatment-resistant schizophrenia, carries two notorious blood-and-heart risks: agranulocytosis (its most famous, covered separately on this page) and myocarditis, an inflammatory heart-muscle reaction concentrated in the first weeks to months of treatment. Myocarditis can be fatal, and because it is most common during the initiation period, regulators require baseline and early monitoring (troponin, CRP, and clinical review of chest pain, tachycardia, or heart failure symptoms). The mechanism is thought to be a hypersensitivity or cytokine-mediated reaction rather than a simple dose effect, which is why it can strike at standard doses. Recognition of the risk has grown steadily, and several health agencies issued specific alerts in the 2010s; the large PRR here reflects both the true risk and the increasingly routine cardiac monitoring around clozapine starts.

Regulatory basis: clozapine labeling (myocarditis/cardiomyopathy warnings); medicines-agency alerts on clozapine myocarditis (e.g. MHRA, 2010s). Established clozapine myocarditis literature.

Bradford Hill viewpoint	Status	Note
Temporality (necessary)	met	Concentrated in the first weeks–months after initiation — a striking temporal signature
Strength	met	PRR 41, one of the largest cardiac signals associated with a psychiatric drug
Consistency	met	Replicated across national pharmacovigilance systems and registries
Specificity	partial	Rarely reported with most other antipsychotics, but not unique to clozapine
Biological gradient	partial	Not clearly dose-related — consistent with an idiosyncratic hypersensitivity reaction
Plausibility	partial	A hypersensitivity/cytokine mechanism is proposed but less precisely defined than for agranulocytosis
Coherence	met	Coheres with the early-monitoring requirement in the label
Experiment	partial	Stopping the drug when myocarditis is suspected improves outcomes; no randomized trial
Analogy	partial	Some analogy to drug-induced hypersensitivity myocarditis (e.g. with other drugs)

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

✗ No ClinicalTrials.gov results-posting trial in the top search hits for clozapine captured myocarditis as a tracked adverse event. Consistent with the mechanism: the reaction is rare, occurs soon after initiation, and typical clozapine trials enroll too few patients (and run safety monitoring that would stop the drug) for an AE table to show it — which is why the signal was identified through post-marketing surveillance and national registries instead.