What Is Serotonin?

Serotonin plays a role in many mental illnesses — and the drugs that are used to treat them.

Medically Reviewed

Serotonin plays a role in many mental illnesses — and the drugs that are used to treat them.

illustration showing seratonin in the brain
A neurotransmitter, serotonin is known for the role it plays in feelings of well-being and happiness.Shutterstock

Serotonin is a hormone and a neurotransmitter that is involved in the function of several different organ systems in the body.

A Happy Chemical: Serotonin and Mood

Serotonin is sometimes known as the happy chemical, because it appears to play an important role in regulating mood, and low levels of serotonin in the brain have been associated with depression. (1)

While there’s a link between low levels of serotonin and depression, it’s not clear whether low serotonin levels cause depression or whether depression causes a drop in serotonin levels. (1)

As a neurotransmitter, serotonin sends messages between nerve cells in the brain. That makes serotonin an important molecule for influencing mental health and brain function.

The Role of Serotonin in Brain Function

In addition to depression, serotonin may play a role in other brain and mental health disorders, including anxiety disorder, obsessive-compulsive disorder (OCD), post-traumatic stress disorder (PTSD), phobias, and even epilepsy.

Serotonin plays an important role in many other body functions, too. It’s involved in appetite and digestion (bowel function and bowel movements), bone health, sex, and sleep.

Serotonin is a precursor to melatonin, a chemical that helps regulate the body’s sleep-wake cycle. (2) Certain antidepressants that raise serotonin levels have been associated with sexual dysfunction.

Too high or too low levels of serotonin have been linked to diseases such as irritable bowel syndrome (IBS), heart disease, and osteoporosis  a disease that weakens the bones — according to an article published in April 2016 in the journal Cell. (3)

Gut Bacteria and Serotonin Production

In recent years, scientists have found that gut bacteria help to produce serotonin and that most of the body’s supply of serotonin can actually be found in the lining of the stomach and intestines. (3)

It’s not clear yet whether — or how — altered serotonin levels in the gut influence brain activity. Some researchers have postulated that serotonin in the gut may stimulate the vagus nerve, the long nerve that connects the digestive tract to the brain. (4)

Selective Serotonin Reuptake Inhibitors (SSRIs)

Selective serotonin reuptake inhibitors, or SSRIs, are a class of drugs used to treat depression and anxiety. They’re the most popular class of antidepressants. (5)

Commonly prescribed SSRIs include:

How Do SSRI Drugs Work?

SSRIs are thought to work by increasing serotonin levels in the brain. SSRIs do this by blocking the absorption of serotonin by nerve cells, keeping more of it available for passing along further messages between nerve cells in the brain.

Other groups of antidepressants, called serotonin and norepinephrine reuptake inhibitors (SNRIs) and serotonin-norepinephrine-dopamine reuptake inhibitors (SNDRIs), block the absorption of serotonin and the other neurotransmitters norepinephrine and dopamine. (5)

SSRIs are sometimes called second generation antidepressants. In general, these antidepressants have fewer side effects than older tricyclic antidepressants.

Still, there are still several common side effects associated with SSRI use. These may include: (6)

  • Dry mouth
  • Nausea
  • Insomnia
  • Headache
  • Nervousness (jitters)
  • Weight gain
  • Diarrhea
  • Sweating
  • Sexual dysfunction

Some people — especially children, teens, and young adults — may have an increase in suicidal thoughts while taking SSRIs.

These side effects are most common when people first start to take SSRIs — or when they change a dose — and tend to lessen over time.

Who Benefits From Use of SSRIs?

SSRIs appear to work best for people with major or severe depression. A review of studies published in April 2018 in the journal The Lancet found that most antidepressants, including commonly prescribed SSRIs, offered a modest benefit over a placebo treatment for people with major depressive disorder. (7) According to the U.S. National Library of Medicine, antidepressants, including SSRIs, helped to relieve depression symptoms in about 20 percent of people. (8)

The benefits of SSRIs for people with mild to moderate depression remain unclear.

Some researchers have shown that SSRIs are most effective when they are combined with talk or behavior therapies that help depression sufferers learn new strategies for coping with troublesome thoughts. A study published in June 2018 in the journal Nature Communications suggested that serotonin may help to speed learning, which could help to explain these findings. (9)

Serotonin Foods and Supplements

Serotonin isn’t found in foods, but its precursor, tryptophan, is. Tryptophan is an essential amino acid that is important in the production of serotonin. Amino acids are the building blocks of protein.

Scientists have shown that the tryptophan in the diet is linked to levels of serotonin in the brain, with lower amounts of dietary tryptophan causing brain levels of serotonin to drop, according to a review published in January 2016 in the journal Nutrients. (10)

Tryptophan is present in most protein-rich foods. Foods high in tryptophan include:

  • Eggs
  • Nuts and seeds
  • Turkey and other poultry
  • Soy foods
  • Cheese
  • Seaweed

Some studies have proposed that eating tryptophan-rich foods may increase levels of serotonin in the brain and help treat depression symptoms. Other studies have found no correlation between tryptophan-rich foods or supplements and depression symptoms, according to a review published in January 2016 in the journal Current Opinion in Clinical Nutrition and Metabolic Care. (11)

Most research indicates that any serotonin boost you might get from eating high-tryptophan foods is probably small. That’s because foods rich in tryptophan tend to be rich in other amino acids as well — and these molecules all have to compete with one another to be absorbed into the brain. (11)

The dietary supplement 5-HTP (5-hydroxytryptophan), a chemical by-product of tryptophan, also helps to increase serotonin production in the brain. (12)

According to the Natural Medicines Comprehensive Database, the dietary supplement may be effective in reducing symptoms of depression in some people. (12)

But larger studies are needed to prove that 5-HTP is safe and effective, and to date 5-HTP has not been approved by the Food and Drug Administration (FDA) as a treatment for depression.

In addition, 5-HTP should not be taken with antidepressant drugs or other depression medication. Many drugs taken for depression or anxiety also raise serotonin levels, and increasing serotonin levels too much can cause serious side effects, including heart problems. (12)

Serotonin Syndrome

Serotonin syndrome, also called serotonin toxicity, is a rare but potentially life-threatening condition that can happen when serotonin levels are too high. (13)

Serotonin syndrome is most likely to occur when starting an antidepressant medication, increasing the dosage of an antidepressant medication, or when two drugs that raise the body’s levels of serotonin are taken at the same time, causing too much serotonin to accumulate in the brain.

Serotonin-raising medicines include: (14)

If you take any of these medicines, be sure to read the packaging labels for warnings about the potential risks of serotonin syndrome. Talk to your doctor if you have any concerns.

Symptoms of serotonin syndrome usually occur within minutes to hours. They may include: (14)

  • Rapid heartbeat or fast pulse
  • High blood pressure
  • Heavy sweating
  • Rapid breathing
  • Agitation or restlessness
  • Hot, dry skin
  • Shivering, goose bumps
  • Headache
  • Diarrhea
  • Dilated pupils
  • Nausea and vomiting
  • Rigid muscles, twitching
  • Loss of coordination
  • Confusion
  • Hallucinations

Call your healthcare provider immediately if you think you may be experiencing symptoms of serotonin syndrome. Symptoms may vary from mild to severe. Patients with severe serotonin syndrome may require hospitalization in an intensive care unit.

Editorial Sources and Fact-Checking


  1. Serotonin. Encyclopaedia Britannica.
  2. The Pineal Gland and Melatonin. Colorado State University.
  3. Yano JM, Yu K, Donaldson GP, et al. Indigenous Bacteria From the Gut Microbiota Regulate Host Serotonin Biosynthesis. Cell. 2015.
  4. A Second Brain: How Microbes in Your Gut May Affect Your Body and Mind. Harvard University Graduate School of Arts and Sciences. August 22, 2016.
  5. Mental Health Medications. National Institute of Mental Health. October 2016.
  6. Selective Serotonin Reuptake Inhibitors (SSRIs). Mayo Clinic. May 17, 2018.
  7. Cipriani A, et al. Comparative Efficacy and Acceptability of 21 Antidepressant Drugs for the Acute Treatment of Adults With Major Depressive Disorder: A Systematic Review and Network Meta-Analysis. The Lancet. 2018.
  8. Depression: How Effective Are Antidepressants? U.S. National Library of Medicine. January 12, 2017.
  9. Iigaya K, Fonseca MS, Murakami M, et al. An Effect of Serotonergic Stimulation on Learning Rates for Rewards Apparent After Long Intertrial Intervals. Nature Communications. 2018.
  10. Jenkins TA, Nguyen JCD, Polglaze KE, et al. Influence of Tryptophan and Serotonin on Mood and Cognition With a Possible Role of the Gut-Brain Axis. Nutrients. 2016.
  11. Strasser B, Gostner JM, Fuchs D. Mood, Food and Cognition: Role of Tryptophan and Serotonin. Current Opinion in Clinical Nutrition and Metabolic Care. 2016.
  12. 5-HTP. MedlinePlus. November 30, 2017.
  13. Boyer EW. Serotonin Syndrome (Serotonin Toxicity). UpToDate. March 12, 2018.
  14. Serotonin Syndrome. MedlinePlus.
  15. St. John’s Wort and Depression: In Depth. National Center for Complementary and Integrative Health. December 2017.


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