Causality assessment · Causality: well established

Venlafaxine (serotonergic antidepressants) and Serotonin syndrome: is the FAERS signal real?

n=2,520 co-reports · PRR 24.1 · ROR 24.5

Venlafaxine (serotonergic antidepressants) 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

Venlafaxine (serotonergic antidepressants) ↔ Serotonin syndrome

n=2,520 co-reports · PRR 24.1 · ROR 24.5

Serotonin syndrome is a predictable, dose-related toxidrome — agitation, clonus, hyperthermia, autonomic instability — produced by excess central serotonergic activity, typically when a serotonergic drug is combined with another (an MAOI, tramadol, triptan, linezolid, lithium, or another antidepressant) or when doses are escalated. Venlafaxine is a potent serotonin-reuptake inhibitor at lower doses, and its risk rises if larger doses begin to add noradrenergic effects; like all serotonergic antidepressants, it carries a labeling warning about the syndrome and about combinations to avoid. The mechanism is about as well understood as any in psychopharmacology, which is why the diagnosis is made clinically and managed by stopping the offending agents rather than through trial data. The large PRR reflects the scale of antidepressant prescribing and the syndrome's seriousness.

Regulatory basis: venlafaxine/SSRI/SNRI class labeling (serotonin syndrome warning; MAOI and serotonergic-combination cautions). Established serotonin-syndrome literature (Hunter criteria).

Bradford Hill viewpoint	Status	Note
Temporality (necessary)	met	Onset follows starting, escalating, or adding a serotonergic drug
Strength	met	PRR 24 with a large co-report count
Consistency	met	Reproduced across the whole serotonergic class and in the literature
Specificity	partial	A class-wide effect, not unique to venlafaxine
Biological gradient	met	Clearly dose-related and combination-related
Plausibility	met	Excess central serotonin is the established cause
Coherence	met	Coheres with the Hunter criteria and the class labeling
Experiment	met	Withdrawing the offending agents reliably resolves it — a consistent dechallenge
Analogy	met	Directly analogous across SSRIs, SNRIs, MAOIs, and tramadol

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

✗ No ClinicalTrials.gov results-posting trial in the top search hits for venlafaxine captured serotonin syndrome as a tracked adverse event. The syndrome is an acute, combination-related toxidrome that would rarely be captured as a coded trial event; its evidence base is clinical and pharmacovigilance-based (the Hunter criteria), not randomized.