Serotonin Syndrome: How to Recognize It and What to Do

Serotonin syndrome is what happens when there is too much serotonin activity in the nervous system, and it runs from mild and self-resolving to fatal. The sign that tells it apart from everything else is clonus: rhythmic involuntary muscle contractions, most easily seen as an ankle that beats or pulses when you flex the foot. Severe cases reach temperatures above 41°C (106°F) with rigid muscles, organ failure and death. It is not rare at festivals, and catching it early is the difference between a medical tent and an ICU.

What it looks like

People who have had it describe an ankle that “beats” when flexed, eyes that jitter sideways on their own, or legs that will not stop moving. That is clonus, and it is not the same as jaw clenching or the general restlessness of a stimulant. Clonus that happens with no trigger at all is a sign of severe toxicity.

Clinicians use the Hunter Serotonin Toxicity Criteria, validated against toxicology case records and more accurate than the older Sternbach criteria. Someone meets them if they have taken a serotonergic drug and have any one of:

• Spontaneous clonus
• Inducible clonus (from flexing the ankle) plus agitation or heavy sweating
• Ocular clonus (eyes drifting and beating rhythmically) plus agitation or heavy sweating
• Tremor plus hyperreflexia
• Muscle rigidity plus a temperature above 38°C (100.4°F) plus clonus

Symptoms fall into three groups, and you do not need all three.

Neuromuscular signs are the specific ones: clonus, hyperreflexia, myoclonus (brief involuntary jerks), tremor, and in severe cases rigidity. Very little else produces clonus.

Autonomic instability means hyperthermia, fast heart rate, high blood pressure, heavy sweating, dilated pupils, diarrhea. All of that overlaps with other emergencies, which is exactly why the neuromuscular findings carry the diagnosis.

Altered mental status means agitation, anxiety, confusion, disorientation. Least specific on its own, part of the picture together with the rest.

Severity is a spectrum. Mild looks like tremor, tachycardia, sweating and wide pupils, and it often clears within 24 hours once the drug is out. Moderate means temperatures up to 40°C (104°F), real agitation, clonus and hyperreflexia, and it needs medical attention. Severe means above 41°C (106°F), extreme rigidity, rhabdomyolysis with the kidney damage that follows it, and renal failure. That is an intensive care problem and it can kill.

What to do

Call 911, or your country’s emergency number, if any of these are true:

• Temperature above 39°C (102°F)
• Visible muscle twitching, or an ankle that pulses when you flex it
• Significant confusion or agitation
• Rapid or irregular heart rate
• They are getting worse rather than better

While you wait:

Stop any further serotonergic substances going in.

Cool them actively if they are hot. Ice packs into the neck, armpits and groin, cool water on the skin. Do not give Tylenol or ibuprofen for the fever. The heat here is generated by uncontrolled muscle activity, not by the prostaglandin mechanism that antipyretics act on, so they do nothing.

Benzodiazepines (diazepam, lorazepam) are the first-line treatment for the agitation, muscle overactivity and autonomic instability. In the field that means getting the person to a medical tent or an ambulance quickly rather than dosing them yourself.

Cyproheptadine, a 5-HT2A antagonist that blocks the receptor serotonin is overstimulating, is the antidote used in moderate cases, typically 12 mg to start and then 2 mg every two hours by mouth or nasogastric tube. That is a hospital treatment, not a field one, but it is worth knowing it exists so you know what they are doing.

Two things not to do. Do not give antipsychotics, which get reached for empirically when someone is agitated but do not block 5-HT2 adequately and carry QTc prolongation risk. And do not leave a symptomatic person alone, because this deteriorates fast.

It is not heat stroke, and it is not a bad trip

Heat stroke shares the high temperature, confusion and cardiovascular strain. It does not produce clonus, hyperreflexia or muscle jerking. Someone who is just hot and dehydrated will not have a pulsing ankle when you flex their foot.

Neuroleptic malignant syndrome comes from antipsychotics and gets confused with this constantly, since both bring hyperthermia and muscle findings. NMS builds over days where serotonin syndrome builds over hours, and NMS produces lead-pipe rigidity with slowed or absent reflexes rather than hyperreflexia. Clonus is the divider: NMS does not cause it.

Anticholinergic poisoning, from a diphenhydramine overdose or certain plants, brings agitation, wide pupils, a fast heart and a high temperature too. Look at the skin. Anticholinergic toxidrome is flushed and dry with absent bowel sounds. Serotonin syndrome is soaked in sweat with active, often hyperactive bowel sounds.

Which combinations actually cause it

MDMA plus an MAOI is the dangerous one, with a documented fatality record. MAO enzymes are what clear serotonin after release, so with an MAOI on board the serotonin MDMA dumps into the synapse has nowhere to go. That means phenelzine (Nardil), tranylcypromine (Parnate), selegiline (Emsam) and moclobemide, and also linezolid, an antibiotic with MAOI activity that people do not know they are taking a risk with. Multiple fatal MDMA and moclobemide cases went to the Victorian State Coroner. If you take an MAOI, MDMA is an absolute contraindication.

MDMA plus an SSRI or SNRI is more complicated than its reputation. SSRIs blunt MDMA rather than amplifying it, so serotonin syndrome from that pair alone is less common than people fear. The risk arrives when someone keeps taking more MDMA to punch through the blockade, or when a third serotonergic thing is in the mix.

MDMA plus lithium stacks two mechanisms that both increase serotonin release.

Then the additive ones. Tramadol is a weak serotonin reuptake inhibitor on top of being an opioid. St. John’s Wort gets sold as a gentle natural supplement and behaves pharmacologically like a mild SSRI, with multiple case reports of serotonin syndrome symptoms alongside serotonergic drugs. Triptans for migraine are 5-HT1B/1D agonists, and the FDA’s 2006 advisory about triptans with SSRIs overstated the absolute risk, but they do add to the total load if MDMA is also involved.

Stopping your SSRI for the weekend does not work

This is the misconception most likely to hurt someone. Skipping doses for a day or two before an event does not clear the interaction, and with fluoxetine it is not close.

Fluoxetine’s active metabolite, norfluoxetine, has a half-life of 4 to 16 days. Stopping on Thursday for a Saturday event leaves the drug fully active, the serotonin transporter still blocked, the interaction fully in place.

Farre and colleagues gave healthy volunteers SSRI pretreatment before MDMA in 2007 and confirmed both halves of the problem: SSRIs substantially blunt MDMA’s subjective effects, while the pharmacokinetic interaction remains, including higher MDMA plasma concentrations from CYP2D6 inhibition. The experience gets worse and the exposure does not go away.

That combination is what makes this dangerous rather than merely disappointing. Someone who believes their SSRI has washed out feels very little, takes more, and now has a large MDMA dose with the transporter still blocked and plasma levels running higher than normal. That is a worse position than simply using MDMA would have been. The full pharmacology is in our MDMA and SSRI interaction guide.

If you are weighing up a specific combination before the night, the drug interaction checker covers the common ones, and the MDMA harm reduction guide has dosing and risk in full. If you are reading this because someone in the room has clonus and a fever, stop reading and call for help.

Sources

Dunkley EJC, et al. “The Hunter Serotonin Toxicity Criteria: simple and accurate diagnostic decision rules for serotonin toxicity.” QJM. 2003;96(9):635-42. PMID 12925718

Boyer EW, Shannon M. “The serotonin syndrome.” N Engl J Med. 2005;352(11):1112-20. PMID 15784664

Farre M, et al. “Pharmacological interaction between 3,4-methylenedioxymethamphetamine (ecstasy, MDMA) and paroxetine: pharmacological effects and pharmacokinetics.” J Pharmacol Exp Ther. 2007;323(3):954-62. PMID 17890444