Too Much Serotonin: Causes, Symptoms, Solutions

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Last updated February 20, 2023.
Edited and medically reviewed by Patrick Alban, DC. Written by Deane Alban.

An excess of serotonin is linked to a long list of symptoms and some mental health conditions, but a high serotonin level can be counteracted naturally.

Serotonin is one of the most widely studied neurotransmitters — chemicals used by brain cells to communicate with each other.

It exhibits a wide range of functions in both the brain and the body, especially in the central nervous system and the gastrointestinal tract.

But mostly, serotonin is known as the “happy molecule” for the important role it plays in maintaining a positive mood.

A deficiency of serotonin is thought to be a major factor in depression, but too much of it can be a very serious health hazard.

If you take any antidepressant drugs or supplements, you need to be aware of the causes and symptoms linked to an excess of serotonin.

Symptoms of Too Much Serotonin: Mild to Serious

A high level of serotonin leads to excessive nerve cell activity which can cause a wide range of symptoms, from mild to severe. 

" Serotonin syndrome doesn’t “just happen.” It is almost always caused by taking one or more medications or supplements known to artificially increase serotonin.

According to Harvard researcher Datis Kharrazian, PhD, DHSc, author of Why Isn’t My Brain Working?, mild symptoms of too much serotonin include:

  • shyness
  • feeling “not good enough”
  • desiring, yet fearing, social interactions
  • nervousness
  • being easily upset by criticism
  • lack of motivation

More serious symptoms of too much serotonin include: 

  • feeling agitated or restless
  • mental confusion or disorientation
  • headache
  • dizziness
  • increased heart rate or blood pressure
  • dilated pupils
  • goose bumps, sweating, or shivering
  • diarrhea, nausea, or vomiting
  • tremors or twitchy muscles


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When serotonin levels get dangerously high, the condition is referred to as serotonin syndrome or serotonin toxicity.

It can be quite serious, and in 2-12% of cases can be life-threatening. 

Serotonin syndrome doesn’t “just happen.”

It is almost always caused by taking one or more medications or supplements known to artificially increase serotonin.

Symptoms of serotonin syndrome include:

  • high fever
  • irregular heartbeat
  • seizures
  • unconsciousness

Note: If you experience any of these symptoms or suspect you have serotonin syndrome, seek medical attention immediately. 

Antidepressants: The Top Cause of Excessive Serotonin

By far the most common cause of excess serotonin is the use of prescription antidepressants that increase serotonin levels in the brain.

This group of medications includes: 

  • selective serotonin reuptake inhibitors (SSRIs)
  • serotonin norepinephrine reuptake inhibitors (SNRIs)
  • monoamine oxidase inhibitors (MAOIs)
  • tricyclic antidepressants (TCAs)

Even though these drugs are some of the most commonly prescribed drugs in the world, many doctors still aren’t aware of the problems that can result from excess serotonin

You’re at particular risk for high serotonin if you are taking a new antidepressant, increasing your dosage, or are taking an antidepressant along with one or more other drugs known to increase serotonin.

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Other Drugs That Cause High Levels of Serotonin

Besides antidepressants, there are many other drugs that can raise serotonin levels too high, including: 

  • anti-nausea medications
  • cough and cold medications (over-the-counter)
  • migraine medications
  • pain medications
  • psychiatric medications
  • recreational drugs (amphetamines, cocaine, ecstasy, LSD)

Note: To confirm if any drugs you’re now taking can contribute to high serotonin levels, review or download Medications That Raise Serotonin Levels, a comprehensive list of prescription medications compiled for Be Brain Fit by

Supplements That Contribute to High Levels of Serotonin

In addition to drugs, there are also a few nutritional supplements that can cause high levels of serotonin, especially when they are taken with an antidepressant.

This can happen when doctors aren’t aware of interactions, or patients self-prescribe supplements without their doctor’s knowledge.

Three of the most widely used supplements taken for depression are the amino acids tryptophan, 5-HTP, and SAM-e.

Since they all work by increasing serotonin, it is not safe to mix any of these supplements with antidepressant medications. 

It’s also not safe to mix them with each other.

This is concerning when you realize that there are numerous supplements that include two or even all three of these amino acids.


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Many people are tempted to use these supplements to wean themselves off their antidepressant medication, not realizing the hazards of mixing these substances.

A very important point to remember is that some antidepressants have long half-lives.

For example, Prozac interactions can occur for up to 5 weeks after its use is stopped.

So it’s very common for people to start taking a serotonin-boosting supplement not realizing that they still have antidepressants circulating throughout their system.

If you are unsure whether your medications or supplements can be safely mixed, talk to your doctor or pharmacist.

In the meantime, you can get basic guidance from one of the reputable online drug interaction checkers found in our Mental Health Resources Guide.

Medical Causes of Excess of Serotonin

While serotonin-boosting drugs and some supplements are the most common causes of high serotonin levels, they aren’t the only ones.

An excessive level of serotonin can also be caused by a certain gene and a specific health condition.


Methylation is a biochemical process that moves a methyl group (one carbon and three hydrogen molecules) from one substance to another.

This process is required for numerous vital functions that occur in the body, including the formation of neurotransmitters

Some people have a mutation in the gene that controls this process, the MTHFR gene.

This leads to a tendency to overproduce serotonin, causing an excess amount in the brain.

People with this gene usually have high blood levels of copper and low levels of zinc as well. 

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Carcinoid Tumors

This final cause of excess serotonin is rare and is the only disease known to cause serotonin syndrome.

Carcinoid tumors are cancerous tumors found in the stomach, small intestine, appendix, colon, and lungs. 

When located in the intestinal tract, they can cause the release of excess serotonin.

This may seem odd until you realize that up to 95% of your total serotonin is synthesized in your intestines by bacteria(!), and not in your brain. 

These tumors are usually asymptomatic and are discovered only during medical exams or while being tested for other conditions.

Mental Health Implications of Excessive Serotonin

Excess serotonin has been linked to two major mental health conditions.

Too Much Serotonin and Social Anxiety

It’s estimated that 15 million Americans suffer from social anxiety disorder

This condition is characterized by overwhelming fear and self-consciousness in social or performance situations and goes way beyond ordinary shyness.

It’s been thought that social anxiety is caused by a lack of serotonin, so serotonin-boosting antidepressants (SSRIs) are commonly prescribed.

But new research is turning that idea on its head.

By using PET cameras and special tracers, researchers at Sweden’s Uppsala University were able to measure serotonin synthesis in the brains of patients.

The study results, published in the prestigious Journal of the American Medical Association, concluded that individuals with social phobia make too much serotonin and the more serotonin they produce, the more anxious they become. 

The study participants with social phobia produced more serotonin in the amygdala, the brain’s “fear center.”

The results of this study have been called a “major leap forward” in the understanding of mental disorders. 

The Link Between Too Much Serotonin and Autism

High blood levels of serotonin have been linked to autism spectrum disorder.

This biomarker occurs in 25% of all autistic children. 

Interestingly, elevated blood levels of serotonin are also found in immediate family members who are not autistic.

The significance of this association is not clearly understood at this time, but another key to the puzzle may lie in the fact that children exposed to drugs that increase serotonin in utero have an increased risk of developing autism.

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Supplements That Moderate Serotonin Levels

As we’ve seen, having either too much or too little serotonin has profound implications.

Fortunately, there are a handful of supplements that modulate or normalize serotonin production to regulate serotonin levels.

Bacopa (Bacopa monnieri) is a traditional Ayurvedic herb that is at the top of this list.

It is a master neurotransmitter regulator that dials the production of neurotransmitters up or down as needed.

In this way, bacopa brings the levels of serotonin and other major neurotransmitters into balance. 

Both vitamin D and omega-3 fatty acids (fish oil) normalize serotonin levels by regulating the conversion of tryptophan into serotonin. 

Schisandra (Schisandra chinensis) is a traditional Chinese herb that balances the production of both serotonin and stress hormones. 

Other supplements that may help to regulate serotonin synthesis include Ginkgo biloba, Rhodiola rosea, and chamomile (Matricaria recutita). 

[Want to know more about these supplements, including benefits, dosages, and potential side effects and interactions? Search our site to find in-depth articles on most of these supplements.]

Can You Be Tested for a High Level of Serotonin?

When we talk about a high serotonin level, you may get the impression that this is a condition that can be easily measured, but there is no reliable laboratory test for excessive serotonin.

Even the diagnosis of serotonin syndrome is made from patient symptoms and drug use history rather than a blood test. 

While there are tests that measure the amount of serotonin in blood, saliva, or urine, they aren’t as useful as you might think since no correlation has been found between these levels and those in the brain

Additionally, there is no scientifically established standard for a “normal” level of serotonin or any other neurotransmitter. 

So for now, symptoms are the best indicator of your serotonin status.

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