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Pharmacology & Therapeutics

Serotonin syndrome

Excess central serotonergic activity produces neuromuscular excitation, autonomic instability and altered mental state within hours, and the muscle activity itself generates the hyperthermia that kills.

In a nutshell

Serotonin toxicity is excess central serotonergic activity within hours of a drug being added, increased, combined or overdosed, causing clonus, hyperreflexia, autonomic instability and muscle-generated hyperthermia. Stop every serotonergic drug and give a benzodiazepine: diazepam 10 mg or lorazepam 2 mg, orally in mild and intravenously above that.

Classic presentation

Six hours after tramadol is added to sertraline, a patient is agitated and sweating with diarrhoea, a fever and inducible ankle clonus with hyperreflexia.

Key points

  • Serotonin toxicity occurs in about 15% of selective serotonin reuptake inhibitor (SSRI) overdoses. Overdose of a single agent rarely causes severe disease.
  • 60% of cases begin within 6 hours of the new drug or dose change. Neuroleptic malignant syndrome takes days.
  • Large benzodiazepine doses are often needed, for example diazepam 40 mg intravenously or lorazepam 4 mg intravenously.
  • Hunter criteria: sensitivity 84%, specificity 97%, validated in overdose. Mild toxicity may not meet them, so they cannot exclude it.
  • Ocular clonus is a non-directional nystagmus. Look for it, and for ankle clonus, in every suspected case.
  • Severe toxicity is almost always an interaction, classically a monoamine oxidase inhibitor (MAOI) with an SSRI.
  • Grapefruit juice, ondansetron, metoclopramide, chlorphenamine, methylthioninium chloride (methylene blue) and linezolid are the serotonergic agents most often missed.

First-line investigation

No test confirms it. Apply the Hunter criteria at the bedside. Send U&E, creatine kinase (CK), glucose and a venous gas; add FBC, clotting, LFT, calcium, phosphate and magnesium if moderate or severe.

Management

Stop the drug and grade it

  • Withhold every serotonergic drug, including fentanyl patches. ABCDE (airway, breathing, circulation, disability, exposure) assessment. Phone the National Poisons Information Service (NPIS) for anything beyond clear-cut mild toxicity.1,7,2
  • Grade by temperature: mild is afebrile; moderate is 38 to 39.9 degrees Celsius with clonus and agitation; severe is 40 degrees Celsius or above with hypertonicity and obtundation.4,5

Benzodiazepines, cooling and fluid

  • Mild: oral diazepam 10 mg or oral lorazepam 2 mg, up to three doses in 24 hours, then seek specialist input.1,8
  • Moderate or severe: the same drugs intravenously, up to three doses, then specialist input. Larger cumulative doses are often required, so be ready for respiratory depression.1,4,9
  • Begin cooling and give 1 to 2 litres of 0.9% sodium chloride intravenously, refrigerated. Cover the whole body in ice; groin and axilla packs alone fail.4,5,1

Serotonin antagonists

  • Cyproheptadine 12 mg orally or by nasogastric tube, then 8 mg every 6 hours, maximum 32 mg daily. Off label: the BNF monograph covers allergy only.1,4,10
  • Or chlorpromazine 25 mg intramuscularly, only after fluid loading because it lowers blood pressure. Olanzapine 5 to 10 mg intramuscularly, repeated once after 2 hours, is an alternative.5,4,1
  • Neither alters mortality and neither is a rescue drug. Do not let either delay cooling, sedation, fluid or airway control.5,6,3

Severe toxicity

  • Aggressive cooling, intravenous benzodiazepine and rapid sequence induction with ongoing paralysis. Thiopentone 3 to 5 mg/kg, avoiding ketamine; rocuronium 1 mg/kg, then atracurium.4
  • Never fentanyl or alfentanil, which are serotonergic. Never suxamethonium, which causes hyperkalaemia in rhabdomyolysis.4,1
  • Do not use dantrolene routinely: NHS guidance conflicts and the BNF has no serotonin indication. Use only after National Poisons Information Service or clinical-toxicology advice.5,4,11,7

Complications

  • Rhabdomyolysis: intravenous fluid. Treat hyperkalaemia on its own pathway. Do not give bicarbonate routinely; use only for a separate specialist-defined indication after toxicology or renal advice.1,4,7
  • Check the glucose: hypoglycaemia is common and may need repeated treatment. Send serial CK, renal function and urine output.4,5

Observe and restart

  • Observe in secondary care for 12 to 24 hours, at least 6 hours in moderate toxicity, and 12 hours after modified-release venlafaxine.2,3
  • Restart one serotonergic drug at a lower dose before discharge with the specialty team. MAOI switches need secondary-care advice and a washout period.1,2,12

Exam traps

  • Clonus and hyperreflexia mean serotonin toxicity. Lead-pipe rigidity with no clonus, days after an antipsychotic, means neuroleptic malignant syndrome.
  • Antipyretics have no place: the heat is generated by muscle, not by a raised hypothalamic set point.
  • Suxamethonium is prohibited at rapid sequence induction because rhabdomyolysis makes it cause hyperkalaemia. Rocuronium is preferred.
  • Fentanyl and alfentanil are themselves serotonergic, so they must not be used to induce these patients.
  • Cyproheptadine is oral only and is unlikely to work after activated charcoal, so it has little use in severe toxicity.
  • The BNF cyproheptadine monograph covers allergy only. The serotonin-syndrome regimen is off label and comes from NHS emergency guidelines.
  • Clonus and hyperreflexia are more marked in the legs than the arms, so a normal upper-limb examination proves nothing.

Illustrations

The serotonin-toxicity triadDiagram showing serotonergic exposure leading to neuromuscular hyperactivity, autonomic disturbance and altered mental state, with clonus highlighted as the key examination clue.PassFinals · original
Serotonin toxicity and its dangerous mimicsComparison diagram distinguishing serotonin toxicity from neuroleptic malignant syndrome, anticholinergic toxicity, sympathomimetic toxicity, malignant hyperthermia and infection by onset, tone, reflexes, skin, bowel findings and exposure.PassFinals · original
Severity-based management pathwayFlow diagram from stopping serotonergic exposure and assessing severity through benzodiazepine-led supportive care, active cooling and critical-care escalation; cyproheptadine is shown as a specialist, evidence-limited option rather than a replacement for resuscitation.PassFinals · original

Key sources

  1. NHS Lanarkshire, Serotonin Syndrome - Adult, Version 2, via Right Decisions (Board-wide adult guideline, approved October 2025, review October 2028, endorsed by the Area Drug and Therapeutics Committee. States the severity spectrum table, the benzodiazepine, cyproheptadine, olanzapine, fluid and dantrolene regimens, the rapid sequence induction prohibitions, the ineffectiveness of paracetamol and the 24 to 48 hour resolution of mild cases.)Published 1 Oct 2025
  2. Tees, Esk and Wear Valleys NHS Foundation Trust, Medication Safety Series MSS36 - Serotonin Syndrome, v1 (Approved 25 September 2025, review 1 October 2028. Prints the Hunter Serotonin Toxicity Criteria in full with both temperature branches, the 60% within 6 hours onset figure, resolution within 24 hours of stopping, the 12 to 24 hour secondary-care observation period, the 15% incidence in SSRI overdose and the serotonergic drug table.)Published 25 Sept 2025
  3. Isbister GK, Buckley NA, Whyte IM. Serotonin toxicity: a practical approach to diagnosis and treatment. Medical Journal of Australia 2007;187(6):361-365 (The paper that derived the Hunter Serotonin Toxicity Criteria and the source document cited by the NHS Greater Glasgow and Clyde chart. States that the lower limbs show much greater hyperreflexia and clonus than the upper limbs, that sustained clonus is usually found at the ankle and ocular clonus is a non-directional nystagmus, the 15% incidence in SSRI overdose, the 6 and 12 hour observation periods, the absence of controlled trials of serotonin antagonists, and that oral cyproheptadine is unlikely to work after activated charcoal. Used only for these clinical-sign and observation points, because no UK document states them.)Published 17 Sept 2007
  4. NHS Greater Glasgow and Clyde Emergency Medicine, Treatment of Serotonin Toxicity (One-page severity chart. Source of the temperature bands (moderate 38 to 39.9 degrees Celsius, severe 40 degrees Celsius or above), the initial blood set, the rapid sequence induction prohibitions and agents, the cyproheptadine, chlorpromazine, diazepam, lorazepam and dantrolene doses, and the 1.26% sodium bicarbonate volume. The page carries no publication date.)
  5. NHS Greater Glasgow and Clyde, Serotonin Toxicity Recognition and Treatment, Version 4, via Right Decisions (Approved 12 November 2022 by the Glasgow Emergency Medicine Clinical Governance Group; its stated review date of 30 December 2025 has passed. Source of the Hunter criteria sensitivity of 84% and specificity of 97%, the severity definitions, the severity-based investigation sets, the minimum 6-hour observation, the whole-body cooling rule, the statements that cyproheptadine and chlorpromazine are unlicensed for this use and have not altered mortality, that mortality is significantly raised above 40 degrees Celsius, and that dantrolene has no effect in serotonin toxicity.)Published 12 Nov 2022
  6. Chiew AL, Isbister GK. Management of serotonin syndrome (toxicity). British Journal of Clinical Pharmacology 2025;91(3):654-661 (The clinical-toxicology review on which both NHS Lanarkshire and TEWV rest, cited here only as the reason a national guideline does not exist and for the uncertain benefit of serotonin antagonists. The Wiley rendering returns an empty body and could not be opened during this pass, so no number in this chapter rests on it. Note also that the Lanarkshire guideline dates it 2024 in one place and 2025 in another.)Published 1 Mar 2025
  7. National Poisons Information Service, TOXBASE (The NPIS public page. It states that TOXBASE is the primary UK clinical toxicology database, that it is not available for public access and requires registration, and that NPIS runs a 24-hour telephone service for healthcare professionals through a single national number. The database itself sits behind a login and was not consulted for this chapter; nothing here rests on it except the contact route.)
  8. BNF, Diazepam (Corroborates the 10 mg unit dose by intramuscular or slow intravenous injection, repeatable after 4 hours, under severe acute anxiety and acute muscle spasm. The monograph carries no serotonin-syndrome indication.)
  9. BNF, Lorazepam (Source of the parenteral precaution that facilities for managing respiratory depression with mechanical ventilation must be available, and of the Medicines and Healthcare products Regulatory Agency (MHRA) advice that benzodiazepines co-prescribed with opioids cause additive respiratory depression, which matters because tramadol, fentanyl and oxycodone are themselves serotonergic. The monograph carries no serotonin-syndrome indication.)
  10. BNF, Cyproheptadine hydrochloride (Lists symptomatic relief of allergy as the only indication, 4 mg three times daily with a maximum of 32 mg per day. The monograph does not mention serotonin syndrome anywhere, which is why the regimen used in this chapter is off label and is cited to the NHS emergency guidelines that state it.)
  11. BNF, Dantrolene sodium (Gives 2 to 3 mg/kg then 1 mg/kg by rapid intravenous injection to a maximum of 10 mg/kg per course, for malignant hyperthermia only. The monograph does not mention serotonin syndrome. Cited here to show that the maximum is expressed per kilogram, which is why the NHS Lanarkshire printing of a 10 mg maximum is not reproduced.)
  12. NICE NG222, Depression in adults: treatment and management (Published 29 June 2022. There is no NICE guideline for serotonin syndrome, and NG222 does not mention it: the full guideline was read during this pass and contains no reference to serotonin syndrome or serotonin toxicity. Cited only for recommendation 1.9.5, that switching to or from a monoamine oxidase inhibitor must take place in or with advice from secondary care, and recommendation 1.10.6, that switching may need an adequate washout period, particularly to or from irreversible MAOIs or moclobemide.)Published 29 Jun 2022

This page is exam revision material, not medical advice, and must not be used for patient care. Always check drug doses against the BNF and current guidance. Full disclaimer.