[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Conditions](https://durgical.com/en/conditions/) › Serotonin syndrome

[Conditions](https://durgical.com/en/conditions/) · [Psychiatry](https://durgical.com/en/conditions/)

# Serotonin syndrome

Serotonin syndrome is a drug reaction from too much serotonin, often when two serotonergic medicines are combined. It starts fast and can be life-threatening.

Updated 2026-11-01

## Structures involved

Tap one to see what it is and open it in 3D.

- [Medulla oblongata](https://durgical.com/en/anatomy/medulla-oblongata/)
- [Pons](https://durgical.com/en/anatomy/pons/)
- [Midbrain](https://durgical.com/en/anatomy/midbrain/)
- [Hypothalamus](https://durgical.com/en/anatomy/hypothalamus/)

## Why it happens

- [Drugs of abuse: cocaine, opioids, amphetamines, cannabis](https://durgical.com/en/general-pathology/drugs-of-abuse-cocaine-opioids-amphetamines-cannabis/)
- [The main neurotransmitters](https://durgical.com/en/physiology/the-main-neurotransmitters/)
- [Body temperature regulation](https://durgical.com/en/physiology/body-temperature-regulation/)
- [Serotonin and melatonin synthesis](https://durgical.com/en/biochemistry/pathways/serotonin-and-melatonin-synthesis/)

## What it is

A potentially life-threatening drug reaction caused by excess serotonin activity in the central and peripheral [nervous system](https://durgical.com/en/anatomy/system/nervous-system/). It produces a triad of mental status changes, autonomic hyperactivity and neuromuscular hyperactivity (clonus, hyperreflexia), usually within 24 hours of starting, increasing or combining serotonergic drugs.

## Who gets it

Its frequency is rising with the wide use of antidepressants. Mild cases are often missed. It occurs at any age, including after intentional overdose of serotonergic drugs; severe cases mostly involve monoamine oxidase inhibitors combined with another serotonergic agent.

## Causes

- Antidepressants: SSRIs, SNRIs, MAO inhibitors, tricyclics
- Opioids with serotonergic action (tramadol, meperidine, fentanyl, methadone)
- Triptans, linezolid, ondansetron, dextromethorphan, lithium and St John's wort
- Recreational drugs: MDMA (ecstasy), cocaine, amphetamines

## Risk factors

- Combining two or more serotonergic drugs, especially with an MAO inhibitor
- Switching antidepressants without an adequate washout period
- Recent dose increase or overdose
- Drug interactions that slow serotonergic drug metabolism

## Pathophysiology

Serotonin neurons arise in the brainstem raphe nuclei ([midbrain](https://durgical.com/en/anatomy/midbrain/), [pons](https://durgical.com/en/anatomy/pons/) and medulla) and project to the cortex, [hypothalamus](https://durgical.com/en/anatomy/hypothalamus/) and [spinal cord](https://durgical.com/en/anatomy/spinal-cord/). Excess stimulation, mainly of 5-HT2A and 5-HT1A receptors, causes agitation, autonomic overdrive, heat production and neuromuscular hyperexcitability that is most marked in the legs. Severe muscle activity drives hyperthermia, rhabdomyolysis and acidosis.

## Classification

- Mild: tremor, tachycardia, sweating, dilated pupils, mild hyperreflexia
- Moderate: inducible or ocular clonus, fever, agitation, hyperactive bowel sounds
- Severe: temperature above 41 °C, rigidity, shock, delirium

## Symptoms and signs

- Agitation, anxiety, restlessness and confusion
- Tachycardia, hypertension, fever, heavy sweating and dilated pupils
- Inducible or spontaneous clonus (ankle, ocular), hyperreflexia and tremor, more in the lower limbs
- Diarrhoea, hyperactive bowel sounds, shivering
- Rapid onset, usually within hours

## Red flags

- Temperature above 41 °C
- Muscle rigidity, especially of the trunk, which can mask clonus
- Seizures, falling consciousness or arrhythmia
- Hypotension or shock

## Diagnosis

**Clinical**
Clinical diagnosis based on exposure to a serotonergic drug plus typical findings. The Hunter criteria require one of: spontaneous clonus; inducible clonus with agitation or sweating; ocular clonus with agitation or sweating; tremor with hyperreflexia; or hypertonia with temperature above 38 °C and ocular or inducible clonus.

**Tests**
No test confirms it; serotonin levels are not useful. Electrolytes, [kidney](https://durgical.com/en/anatomy/kidney/) function, CK, blood gases and coagulation to detect complications; ECG; toxicology screen and other studies to exclude infection.

**Imaging**
Not needed for diagnosis; brain imaging only to exclude other causes when the picture is atypical.

## Differential diagnosis

- Neuroleptic malignant syndrome (slow onset, lead-pipe rigidity, normal or reduced reflexes)
- Anticholinergic toxicity (dry hot [skin](https://durgical.com/en/anatomy/skin/), absent bowel sounds, normal reflexes)
- Malignant hyperthermia
- Sympathomimetic intoxication or alcohol and sedative withdrawal
- Meningitis, encephalitis, sepsis and thyroid storm

## Goal of treatment

Stop all serotonergic drugs, control agitation and temperature, and support vital functions; most cases resolve within 24–72 hours once the drugs are stopped.

## Treatment

**Acute**
Discontinue every serotonergic agent; intravenous fluids, oxygen, cardiac monitoring and external cooling. Severe hyperthermia needs ICU care with sedation, paralysis with a non-depolarising agent and intubation; antipyretics do not help because the heat comes from muscle activity.

**Medical**
Benzodiazepines for agitation, tremor and muscle activity; cyproheptadine, a serotonin (5-HT2A) antagonist, in moderate cases. Physical restraints are avoided because they worsen muscle activity and hyperthermia.

**Follow-up**
Nursing: temperature, heart rate and blood pressure, level of consciousness, clonus, urine output and colour; medication reconciliation to identify every serotonergic drug.

## Complications

- Rhabdomyolysis and acute kidney injury
- Metabolic acidosis
- Disseminated intravascular coagulation
- Seizures, respiratory failure and death

## Prognosis

Good with early recognition: most cases improve within a day of stopping the drugs. Drugs with long half-lives or irreversible MAO inhibition can prolong the course; severe hyperthermia carries a real risk of death.

## Prevention

- Checking for interactions before combining serotonergic drugs (including opioids, antiemetics and linezolid)
- Adequate washout when switching to or from an MAO inhibitor
- Teaching patients about warning symptoms and herbal products

## Test yourself

Think of the answer, then tap the question to see it.

## What is the classic clinical triad of serotonin syndrome?

Altered mental status, autonomic hyperactivity and neuromuscular abnormalities (clonus, hyperreflexia).

## Which neuromuscular sign is most specific for serotonin syndrome?

Clonus (inducible, spontaneous or ocular), most marked in the lower limbs.

## Which serotonin antagonist is used in moderate serotonin syndrome?

Cyproheptadine, alongside stopping the drugs, benzodiazepines and supportive care.

## Which commonly used analgesic can trigger serotonin syndrome when combined with an SSRI?

Tramadol (also meperidine, fentanyl and methadone).

## Why are antipyretics not effective for the fever of serotonin syndrome?

Because the heat comes from muscle hyperactivity, not from a change in the hypothalamic set point.

[Add to my cards (5)](https://durgical.app/estudiar/?add=/en/conditions/serotonin-syndrome/)

## Related conditions

- [Neuroleptic malignant syndrome](https://durgical.com/en/conditions/neuroleptic-malignant-syndrome/)
- [Obsessive-compulsive disorder (OCD)](https://durgical.com/en/conditions/obsessive-compulsive-disorder/)
- [Acute kidney injury](https://durgical.com/en/conditions/acute-kidney-injury/)
- [Carcinoid syndrome](https://durgical.com/en/conditions/carcinoid-syndrome/)
- [Sepsis and septic shock](https://durgical.com/en/conditions/sepsis-and-septic-shock/)

Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Serotonin syndrome](https://medlineplus.gov/ency/article/007272.htm) (MedlinePlus, US National Library of Medicine, 2026).

[← All conditions](https://durgical.com/en/conditions/)

## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake

Representative model, for educational purposes only.

---
Source: https://durgical.com/en/conditions/serotonin-syndrome/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/conditions/serotonin-syndrome/