Imagine this: You start a new medication or take an extra dose of a medication you’ve used before. Within minutes, you notice a tremor in your hands and maybe some sweating or nausea. Then, things escalate fast. That’s serotonin syndrome, a serious and potentially life-threatening reaction that happens when your body is flooded with serotonin.

"People often think of serotonin as the feel-good hormone. And that’s true. Serotonin is a critical neurotransmitter that helps regulate mood, behavior, memory, and even digestion, appetite and body temperature," said Chau Chu, PharmD, a pharmacy specialist at Cedars-Sinai. "But too much serotonin can overwhelm the brain and body, and that’s when things get dangerous."

Serotonin syndrome (sometimes called serotonin toxicity) happens when your body has too much serotonin, typically due to taking a drug or combination of drugs that affect serotonin levels.

"It can come on suddenly—within minutes or hours of taking a new medication or increasing the dose of a medication you’re already taking," said Rebecca Hedrick, MD, a psychiatrist at Cedars-Sinai. "But it can also happen after months or even years on a stable dose, particularly if you add another medication."

Even something seemingly benign, such as cough syrup or an antihistamine, can cause a reaction in some individuals.

Scientists don’t have a clear understanding of why some people develop serotonin syndrome and others don’t, even when they’re on the same medication. But when it hits, serotonin syndrome produces a constellation of troubling symptoms, from mild (tremor and dilated pupils) to severe (high fever and seizures).

Serotonin syndrome is triggered by medications that target serotonin.

"Antidepressants such as monoamine oxidase inhibitors (MAOIs), selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs) are the most common culprits," said Chu.

Other medications that increase serotonin levels include:

• Stimulants, including prescription medications for ADHD and illicit substances such as MDMA (ecstasy), cocaine and amphetamines
• Opioids, such as fentanyl, methadone, meperidine and tramadol
• Migraine medications, especially triptans
• Anti-nausea medications such as ondansetron (Zofran)
• Herbal supplements, including St. John’s Wort (a key offender), 5-HTP, L-tryptophan, ginseng and nutmeg
• Over-the-counter medications, such as dextromethorphan (DXM), a common ingredient in cough syrup and cold medications

The first and most common symptom of serotonin toxicity is tremor: a shaking or jittery feeling that’s hard to ignore.

Other signs can follow quickly:

• Muscle twitching or jerking
• Restlessness or agitation
• Sweating, nausea, vomiting or diarrhea
• Dilated pupils
• Confusion
• Rapid heart rate or blood pressure changes
• High fever
• Seizures

If you experience a combination of these symptoms—especially after starting or mixing medications—seek medical help immediately.

Anyone who takes substances that affect serotonin levels could develop serotonin syndrome. However, some groups are more vulnerable:

• People over age 60 and under age 25 (age impacts how we metabolize medications)
• People with impaired kidney or liver function
• People who are slow metabolizers based on their genetics

"Most cases of serotonin syndrome happen in patients who are taking two or more drugs that increase serotonin in different ways," said Hedrick. "A common example is a teen taking both a stimulant for ADHD and an SSRI for anxiety or depression."

Even mild cases of serotonin syndrome are rare. The problem is serotonin syndrome is often unrecognized, undiagnosed or mistaken for something else—and severe cases can be deadly if they’re not caught and treated quickly. 

Most mild-to-moderate cases of serotonin syndrome usually resolve within 24 to 72 hours after you stop taking the triggering medication. Severe cases, however, may require hospitalization and ongoing monitoring. Treatment may include:

• Benzodiazepines (like lorazepam or clonazepam) to calm agitation, reduce muscle stiffness and prevent seizures
• Cyproheptadine, a serotonin-blocking agent
• IV fluids to stabilize vital signs

"We usually start with benzodiazepines because they can help ease agitation, relax the mind and muscles, and prevent seizures, the most dangerous piece of serotonin syndrome," Hedrick said.

The best way to prevent serotonin syndrome is to understand your risk and recognize its early symptoms. Here’s how to reduce your risk of excess serotonin: 

• Read all warnings on the drug packaging inserts for your medications, and pay close attention to drugs that boost serotonin levels.
• Avoid taking more than one substance that affects serotonin at a time.
• Use prescription and over-the-counter medications only as instructed, and avoid taking extra doses.
• When you start a new medication, ask your doctor or pharmacist if it affects serotonin levels.
• Be cautious when taking over-the-counter supplements and cold medications.

If you take an SSRI or SNRI, avoid mixing it with serotonin-boosting medications or supplements like St. John’s Wort, cough suppressants (like DXM), and antihistamines, or recreational drugs like MDMA.

"And if you notice unusual symptoms—tremors, confusion or sudden sweating—don’t wait it out," said Chu. "Get medical attention. Serotonin syndrome can get worse before it gets better."