Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
Recognise clonus-based serotonin toxicity, reconstruct interacting exposures, stop serotonergic drugs, and control agitation, hyperthermia and organ injury without confusing it with neuroleptic malignant syndrome.
Serotonin excess can follow therapeutic prescribing when mechanisms are combined, not only deliberate overdose. Monoamine-oxidase inhibition creates particularly severe interactions, including with serotonergic antidepressants, some opioids and linezolid. Pharmacy records may omit over-the-counter cough preparations, St John's wort, recreational substances and recent medicines from another prescriber. Exact ingestion time, formulation, dose, metabolism and renal or hepatic impairment shape duration.
The examination separates serotonin syndrome from other hyperthermic states. Clonus, brisk reflexes and tremor favour serotonin toxicity. Neuroleptic malignant syndrome evolves more slowly with generalised lead-pipe rigidity and hyporeflexia after dopamine blockade. Anticholinergic poisoning causes dry skin and reduced bowel activity rather than sweating and diarrhoea. Sepsis, meningitis, sympathomimetic toxicity, heatstroke, thyrotoxicosis and malignant hyperthermia require parallel consideration. Severe rigidity may eventually obscure clonus, so early findings and drug history matter.
SSRIs or SNRIs combined with MAO inhibition, tramadol, linezolid, lithium or other serotonergic agents can create excessive signalling.
Intentional overdose, rapid increase or reduced clearance can raise serotonergic exposure even without a second medicine.
Enzyme inhibition and long drug half-lives can preserve hazardous concentrations after a medicine appears to have been stopped.
MDMA, dextromethorphan, certain opioids and herbal products may be overlooked unless the exposure history is non-judgemental and complete.
Excess production, release, receptor stimulation or impaired breakdown raises serotonergic activity across central and peripheral nervous systems.
Spinal and brainstem receptor overactivation produces tremor, hyperreflexia and spontaneous, inducible or ocular clonus on examination.
Central and peripheral signalling causes agitation, diaphoresis, mydriasis, tachycardia, pressure change and increased gastrointestinal activity during toxicity.
Severe sustained contraction produces heat, acidosis and rhabdomyolysis that can progress to multi-organ failure without rapid control.
Agitation and tremor begin within hours of adding, increasing or overdosing a serotonergic medicine or combining it with an MAO inhibitor.
Repeated rhythmic ankle movement after sudden dorsiflexion, especially with agitation or diaphoresis, is a high-value sign of serotonin toxicity.
Spontaneous or induced rapid eye oscillation with agitation and sweating supports the diagnosis when limb examination is difficult.
Mydriasis, sweating, tachycardia, blood-pressure elevation, hyperactive bowel sounds, vomiting or diarrhoea accompany hyperreflexia.
Temperature above 40 degrees, sustained rigidity, seizure, acidosis or rapidly rising CK indicates life-threatening toxicity requiring intubation and paralysis.
A several-day onset after dopamine blockade with diffuse rigidity and reduced reflexes favours neuroleptic malignant syndrome rather than early clonus-dominant serotonin toxicity.
Slower evolution with profound rigidity and reduced reflexes after dopamine disruption favours NMS over clonus-predominant serotonin toxicity.
Dry skin and reduced bowel sounds support anticholinergic poisoning, while stimulant toxicity lacks the characteristic clonus pattern.
A preceding psychomotor syndrome with mutism, negativism or posturing supports catatonia, although autonomic and motor overlap can be substantial.
Fever, confusion and shock require infection investigation; a medicine history and clonus should not suppress cultures or antimicrobial treatment when indicated.
Sustained muscle activity causes severe heat, muscle breakdown, hyperkalaemia and kidney injury during severe uncontrolled toxicity.
Extreme serotonergic activation can provoke seizures, delirium, coma and secondary hypoxic brain damage during severe toxicity.
Acidosis, hyperthermia and shock can trigger disseminated coagulation, hepatic injury, respiratory failure and death when untreated.
Agitation, reduced consciousness and sedation increase aspiration and airway risk, while physical restraint worsens heat generation.
Inducible ankle clonus with agitation or sweating is more diagnostically useful than non-specific fever, tachycardia or raised creatine kinase.
A syndrome developing within hours of a serotonergic change fits better than NMS, which more often evolves over several days.
Its active metabolite creates interaction risk for weeks, making a recently stopped prescription relevant to an MAO-inhibitor exposure.
The dangerous temperature comes from sustained muscular activity, so sedation, paralysis and external cooling are the effective controls.
Serotonin concentration does not diagnose tissue toxicity; bedside neuromuscular findings and exposure history drive recognition.
Missing over-the-counter dextromethorphan or illicit serotonergic exposure.
Ignoring a recently stopped long-half-life antidepressant.
Calling hyperreflexia and clonus lead-pipe rigidity.
Using paracetamol as definitive treatment for extreme hyperthermia.
Restraining an agitated patient and increasing muscle heat.
Waiting for a serotonin level that has no diagnostic role.
Giving cyproheptadine while delaying intubation and paralysis.
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Hours after linezolid is added to an SSRI, a patient develops agitation, diaphoresis, diarrhoea and tremor. Which examination finding most strongly supports serotonin syndrome?
National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.
Editorial contact: Doctor’s Passport editorial team. Source-check date is separate from clinical approval. Report a content problem · Content changes
Respiratory depression, aspiration, delirium and prolonged sedation can occur, especially with co-ingested depressants; airway readiness must match dose escalation.
Evidence is limited and anticholinergic sedation and hypotension can complicate assessment; never delay airway control, paralysis or cooling to obtain it.
Paralysis hides seizures and neurological signs and mandates full ventilation, sedation and monitoring; avoid succinylcholine when hyperkalaemia or rhabdomyolysis increases risk.