Serotonin Syndrome — What Every Supplement User Needs to Know

Okay, I’m going to be honest. I used to think things from plants or from health food stores were a lot different than proper medicines. I believed they were milder, safer, and wouldn’t give you the serious difficulties you’d find described in medical publications. But I've discovered that not only is this not correct, sometimes there are potentially lethal things to watch out for.

I recently learned about serotonin syndrome. This post is about a true risk where natural treatments and standard medical treatment meet. If you take supplements for anything and are also on prescribed medication for depression, blood pressure or ADHD you really need to know about this. It isn’t widespread knowledge, but if you do get serotonin syndrome it occurs quickly and unfortunately, it can be fatal.

What serotonin syndrome actually is

Serotonin, a brain chemical, is key to mood and also sleep and appetite. In depression, one common notion is that activity of serotonin in the brain is too low. Most antidepressants function, in part, by increasing availability of serotonin at the nerve junctions where it performs its work.

Here is what most people are unaware of. Now, there is too much serotonin activity. And when you do it really isn’t just feeling somewhat off. It’s called serotonin syndrome and it occurs as a result of too much serotonin activity throughout the brain and central nervous system. NCBI Its symptoms vary from mild side effects to life-threatening toxicity.

Calling it a “syndrome” can make it seem more manageable. A syndrome you muddle through. This is not that.

What it feels like. With mild cases come increased heart rate, shivering, sweat, dilated pupils and twitching, as well as overactive reflexes. Wikipedia Moderate cases also feature severe hyperthermia and hypertension. Severe cases are marked by muscle rigidity, sustained abnormal muscle movements and a sharply surging temperature that can exceed 40 degrees Celsius in hours. PubMed Central The most frequent serious complication, which occurs in about one quarter of severe cases, is the breakdown of muscle tissue that can injure the kidneys.

People have died from this. There are an estimated 7,300 diagnoses in the United States annually and about 100 deaths. According to a systematic review of 56 fatal cases those who fell victim were aged between 18 and 87 years. In 59 percent of those cases, symptoms presented themselves within 24 hours after taking the serotonin-affecting substances.

How it happens

Serotonin syndrome is almost always a product of combination. It typically arises when a new serotonin-affecting substance is added, when a dose is changed, or when two or more substances interact in a way that pushes serotonin activity beyond what the body can regulate. NCBI

The concerning thing for anyone taking supplements is that the trigger does not have to be two prescription drugs. It can be a supplement and a prescription drug. Or two supplements taken together.

Most cases develop within 24 hours of any change in serotonin-affecting substances. NCBI This is not something that creeps up slowly over weeks.

The herbs and supplements involved

St. John's Wort is the headline act here. It raises levels of serotonin and related brain chemicals through the same basic mechanism as many prescription antidepressants. Combining it with an antidepressant can dangerously elevate serotonin levels. PubMed Central New Zealand's medicines regulator reported a case of serotonin syndrome requiring hospitalisation following just one cup of St. John's Wort tea alongside two days of the antidepressant citalopram. US Pharmacist

One cup of tea. Two days of medication. That is worth sitting with for a moment.

5-HTP is another supplement many people reach for when mood is low. It is a compound your body produces from the amino acid tryptophan and it sits one step away from serotonin in the chemical chain. Because antidepressants generally work by increasing serotonin in the brain 5-HTP could combine with these medications to produce dangerously high serotonin concentrations. Poison Control Case reports bear this out. A 33-year-old woman arrived at an emergency room with nausea, diarrhoea, agitation, hallucinations and tremors after taking 5-HTP alongside the antidepressant sertraline. Poison Control

SAMe is a third. It is a naturally occurring compound sold widely as a supplement for depression and joint pain. Because SAMe may also affect serotonin levels combining it with other supplements that do the same should be avoided to reduce the risk of excess serotonin building up in the body. Memorial Sloan Kettering Cancer Center

L-tryptophan, sold as a sleep and mood supplement, is the direct dietary precursor to 5-HTP and serotonin. The risk profile is similar.

The list goes further. A 2025 pharmacological review identified black cohosh, ginseng, Syrian rue, turmeric, rhodiola and ashwagandha as herbs that have been linked to serotonin syndrome when combined with antidepressant medications. MDPI The evidence linking some of these individually to serious cases is thinner than for St. John's Wort or 5-HTP. But the point holds: these are not inert substances. If you take any of them alongside an antidepressant it is worth a conversation with your doctor.

The stacking problem

Most serotonin syndrome cases involve at least one prescription antidepressant. But the biology does not require one. Increased L-tryptophan or 5-HTP intake can elevate serotonin levels independently which calls for caution even in people not on conventional antidepressant medication. MDPI

The more common scenario in supplement circles is probably stacking. Taking 5-HTP alongside SAMe. Adding St. John's Wort to an existing protocol. Combining several mood-supporting herbs without checking whether they share a mechanism. The individual doses might all seem sensible. The combination is the problem.

The diagnosis problem

One of the genuinely tricky aspects of serotonin syndrome is that it can look like other things. It may go unrecognised because it can be mistaken for a viral illness, anxiety, a neurological problem or simply a worsening of an existing condition. Wikipedia There is no blood test that confirms it. Diagnosis is based on the patient's history and physical examination using criteria called the Hunter Serotonin Toxicity Criteria which have 84% sensitivity and 97% specificity for moderate and severe cases. Wiley Online Library

This matters because the history includes supplements. Investigations should include detailed questions about all prescription drugs, over-the-counter medications, recreational substances and dietary supplements as all of these have been implicated in serotonin syndrome. Wikipedia If you are taken to hospital in this state tell them everything you have been taking. Including the herbal stuff. Especially the herbal stuff.

The bottom line

The evidence points to St. John's Wort and 5-HTP as the highest-risk supplements here with the greatest danger arising when either is combined with a prescription antidepressant, particularly SSRIs or MAOIs. If you take any antidepressant medication do not combine it with either of these without specific guidance from your doctor. That conversation needs to include your full supplement list, not just the prescription.

If you are not on prescription antidepressants but are taking multiple mood-supporting supplements together be cautious about combining anything serotonin-affecting. 5-HTP and SAMe together, for example, could push things further than intended.

And if you or someone nearby develops a sudden racing heart, muscle twitching, agitation and overheating together, shortly after starting or changing serotonin-affecting substances, treat it as a medical emergency. The condition responds well to treatment when it is caught early. The key word there is early.

Citations

1. Mikkelsen N, Damkier P, Pedersen SA. Serotonin syndrome: a focused review. Basic & Clinical Pharmacology & Toxicology. 2023. https://onlinelibrary.wiley.com/doi/10.1111/bcpt.13912
2. Brown TM, Skop BP, Mareth TR. St. John's Wort: A Systematic Review of Adverse Effects and Drug Interactions for the Consultation Psychiatrist. Psychosomatics. https://www.sciencedirect.com/science/article/abs/pii/S0033318203702680
3. Peterson B, Nguyen H. St. John's Wort. StatPearls. 2023. https://www.ncbi.nlm.nih.gov/books/NBK557465/
4. Chiew AL, Isbister GK. Management of serotonin syndrome (toxicity). British Journal of Clinical Pharmacology. 2024. https://pmc.ncbi.nlm.nih.gov/articles/PMC11862804/
5. Fulgencio-Sánchez M et al. Pharmacoepidemiological data on drug-herb interactions: serotonin syndrome, arrhythmias and the emerging role of artificial intelligence. 2025. https://www.mdpi.com/2813-0618/4/4/22
6. Serotonin Syndrome. StatPearls. Updated March 2024. https://www.ncbi.nlm.nih.gov/books/NBK482377/
7. Dunkley EJ et al. The Hunter Serotonin Toxicity Criteria. QJM. 2003. https://pubmed.ncbi.nlm.nih.gov/12925718/
8. Turner EH et al. Serotonin a la carte: supplementation with the serotonin precursor 5-hydroxytryptophan. Pharmacology & Therapeutics. 2006. https://www.sciencedirect.com/science/article/abs/pii/S016372580500135X
9. 5-HTP safety concerns. Poison Control. https://www.poison.org/articles/5htp-safety-concerns-173
10. Karbachova A et al. Dietary supplement-drug interaction-induced serotonin syndrome progressing to acute compartment syndrome. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC5580516/
11. Edinoff AN et al. Fatal serotonin syndrome: a systematic review of 56 cases. PubMed. 2020. https://pubmed.ncbi.nlm.nih.gov/33196298/
12. Serotonin syndrome. Pharmacy Times. https://www.pharmacytimes.com/view/surprising-drugs-that-can-cause-serotonin-syndrome