Dopamine isn’t pleasure, and depression isn’t a serotonin shortage

More than 90 percent of the serotonin in your body is made in your gut. Almost none of that serotonin can cross into your brain.

The first fact appears in a 2015 paper in the journal Cell, the second in biology reviews since, and together they undo a surprising amount of popular brain-chemistry advice. They explain why a urine sample cannot reveal that your brain is “low in serotonin.” They also set up a bigger correction: the old description of depression as a “chemical imbalance” has not held up the way it was sold.

Neurotransmitters are the chemical messengers nerve cells use to signal one another. Two of them, dopamine and serotonin, have become something like brand names, attached to phone habits, diets, supplements and lab tests. Each slogan gets something wrong, and each correction is more useful than the slogan.

Dopamine tracks surprise, not pleasure

“Dopamine equals pleasure” may be the most repeated line in pop neuroscience. The research on dopamine says something different.

Recordings from monkeys in the 1990s showed that midbrain dopamine neurons behave like a prediction system. A 1997 paper in Science by Wolfram Schultz and colleagues described their output as signaling changes or errors in predictions about rewards. In a 2016 review, Schultz summarized the pattern: most of these neurons fire more when a reward is better than expected, stay at their usual rate when a reward arrives exactly as predicted, and dip below it when a reward is worse than expected.

Picture a coworker who brings donuts one Friday. It is a pleasant surprise. By the fourth Friday in a row, the donuts are expected, and in this model a fully expected reward barely moves these neurons. If the box is empty on the fifth Friday, their activity drops. What changed is the gap between what you expected and what you got.

Pleasure itself seems to run partly on other wiring. In a 2015 review in Neuron, Kent Berridge of the University of Michigan and Morten Kringelbach separated wanting a reward from liking it. Wanting, they wrote, comes from a large, distributed brain system. Liking is produced in a smaller set of what they call hedonic hot spots. They went further: some of the best-known textbook candidates for the brain’s pleasure generator, including the mesolimbic dopamine system, “may not generate pleasure after all.”

So dopamine looks more like a teaching and motivation signal than a pleasure chemical.

Two fads built on the slogan

The “dopamine hit from your phone” line borrows the wrong half of this story. If dopamine responds most to rewards you cannot predict, the more relevant feature of notifications is that you never know which one will matter, not that each feels good. What phones and alerts do to attention is weighed in our report on digital distractions.

The “dopamine fast” took the misreading further. According to a 2020 Harvard Health Publishing article by Peter Grinspoon, MD, the idea came from Cameron Sepah, whom the article describes as a California psychiatrist, as a method rooted in cognitive behavioral therapy for cutting back on compulsive habits such as gambling, emotional eating or excessive gaming. Grinspoon reports that Sepah called the name catchy and said it was not to be taken literally.

Some followers treated it as a literal reset, avoiding food, exercise, music and even conversation to replenish dopamine. Grinspoon’s point was that dopamine does not fall when you avoid pleasant things, so a fast cannot lower it. The method was behavioral from the start, and the dopamine in its name was a label, not a mechanism.

Serotonin has two separate systems, and only one is in your brain

A 2019 review by Michael Bader in Pharmacology & Therapeutics described two independent serotonin systems in the body, each built by a different version of the enzyme that makes serotonin. One operates in the brain. The other operates in the gut and the rest of the body, and it stocks platelets, the blood cells involved in clotting. Bader wrote that the duality is enforced by the blood-brain barrier, the tightly sealed lining of the brain’s blood vessels, which serotonin cannot pass.

A 2020 review from Flinders University in Australia made the same point: because serotonin cannot readily cross that barrier, the brain’s supply and the body’s supply are anatomically separated. It added that almost all serotonin outside the brain comes from specialized cells lining the gut, called enterochromaffin cells. The 90 percent figure appears as background in the introduction of the 2015 Cell paper, from a California Institute of Technology team; the study itself was mostly in mice and showed how certain gut bacteria raise serotonin production in those cells.

Gut serotonin does real work, helping move food through the intestines and helping platelets form clots. It does not travel to the brain to set your mood. So “most serotonin is in the gut, therefore gut health is mood health” skips a step. Any link between gut and mood has to run through other routes and stand on its own evidence.

The chemical-imbalance story, and what replaced it

Many people have heard depression explained as a shortage of serotonin. A survey of 1,219 college students, published in 2025 in Frontiers in Psychology by a team led from the University of Michigan, asked where they had heard it. Students had met the explanation in classrooms, online and from friends. But only hearing it from a health care provider independently predicted believing it.

What a 2022 review found

In 2022 a team led by Joanna Moncrieff of University College London published an umbrella review, a review of reviews, in Molecular Psychiatry. It drew on 17 studies covering the main lines of serotonin research. Among the findings:

• A serotonin breakdown product measured in body fluids showed no association with depression.
• Blood serotonin was not related to depression, though lower levels were associated with taking antidepressants.
• The two largest genetic studies, of 115,257 and 43,165 people, found no link between the serotonin transporter gene and depression, and no interaction between that gene, life stress and depression.
• Experiments that temporarily deplete tryptophan, serotonin’s raw material, did not lower mood in most healthy volunteers, with weak evidence of an effect in people with a family history of depression.

The authors concluded that the evidence gave no support to the idea that depression is caused by lowered serotonin. They also reported some evidence that long-term antidepressant use may itself lower serotonin.

The 35-author reply

In 2023, 35 researchers replied in the same journal with a commentary whose title called the review a leaky umbrella of little value. The group, led by Sameer Jauhar of King’s College London with Philip Cowen of Oxford as senior author, called the review’s conclusion overstated. They cited what they described as weak review methods, selective reporting, oversimplification and errors in interpreting drug research, and pointed to tryptophan-depletion and brain-imaging studies as evidence that the serotonin system is involved in depression.

The two sides overlap more than headlines suggested. The reply argued that serotonin is involved in depression, which is a weaker claim than the old slogan that depression is a serotonin shortage. Graded on this library’s evidence scale, “depression is caused by low serotonin” is tested and not supported, while “the serotonin system is involved in depression” is positive evidence with a contested interpretation.

The National Institute of Mental Health’s depression page, last reviewed in December 2024, does not mention serotonin or a chemical imbalance at all. It says research points to genetic, biological, environmental and psychological factors.

What it does not mean

One conclusion does not follow: that antidepressants do not work. The 2022 review and the reply were about what causes depression. Whether a treatment helps is measured in treatment trials, a separate body of evidence that neither paper set out to judge. Whether a particular antidepressant is helping, and whether to keep taking it, is a question for the person who prescribed it, not one to settle from a headline.

If you are struggling with depression or having thoughts of suicide, the 988 Suicide & Crisis Lifeline takes calls and texts at 988 and chats at 988lifeline.org, around the clock, free and confidential. In a life-threatening emergency, call 911.

Why a urine test cannot read your brain chemistry

Some clinics and online sellers offer urine or blood “neurotransmitter panels,” sometimes paired with supplement plans to correct the results. The biology above shows the core problem.

Blood and urine serotonin mostly reflect the gut. Platelets carry gut-made serotonin, and the blood-brain barrier keeps the brain’s supply separate.

The kidney makes its own dopamine. A 2015 review by Vanderbilt University researchers described a dopamine system inside the kidney, separate from any nerve input, that the kidney uses to regulate how much salt and water the body excretes. Dopamine inside the kidney can reach concentrations far above those in the bloodstream, and the kidney makes more of it when dietary salt rises. That means a urine dopamine reading is mixed up with kidney work that has nothing to do with mood.

Even supporters of urine testing concede the gap. A 2023 study in PLOS One, by two authors affiliated with a Japanese commercial laboratory company, measured six urinary neurotransmitters in 100 healthy volunteers and found lower urinary dopamine in the group reporting more job stress. Hopeful about a future screening test, the authors still wrote: “Although urinary neurotransmitter levels may not always reflect those in the brain, we hypothesize that they can still serve as potential biomarkers…” Their limitations section noted that these chemicals are also made in the kidneys and gut, that most cannot cross the blood-brain barrier, and that further validation is needed before the approach could serve as a diagnostic tool.

That is the favorable end of the evidence: one cross-sectional study, from a company in the testing business, comparing group averages. It stops well short of what a consumer test implies, a reading of one person’s brain chemistry. For that purpose these panels sit in the insufficient-evidence category, because the validation their own advocates call for has not been done.

A certified laboratory can measure urinary dopamine precisely, and the number can still say little about your brain. The regulatory plumbing behind consumer-ordered lab work is laid out in our direct-to-consumer testing piece.

The two real risks next to the myths

The slogans are mostly harmless trivia. The real hazards sit beside them.

Serotonin syndrome

The National Library of Medicine’s MedlinePlus describes serotonin syndrome as a potentially life-threatening reaction to certain medicines that leaves the body with too much serotonin. It most often follows combinations, such as an SSRI or SNRI antidepressant taken with a triptan migraine drug, and some street drugs can contribute. Symptoms can begin within minutes to hours, and the risk is highest when a medicine is started or its dose is raised. Signs include agitation, a fast heartbeat, high body temperature, heavy sweating, muscle spasms or overactive reflexes, diarrhea and confusion.

The National Institute of Mental Health warns that combining antidepressants with other serotonin-acting medicines or supplements, naming St. John’s wort, can cause a rare but life-threatening illness. St. John’s wort carries a second problem, according to the National Center for Complementary and Integrative Health: it can weaken many other drugs, including birth control pills, warfarin and some HIV and cancer medicines.

So “boosts serotonin” on a label reads best as a caution for anyone already taking a serotonin-acting medicine, not as a selling point. Whether a particular combination is safe is a call for a prescriber or pharmacist who sees the full medication list. The ways supplements and drugs collide are mapped in our guide to supplement-medication interactions, while whether any mood supplement works on its own terms is reserved for our supplement reports.

Stopping an antidepressant

Discontinuation symptoms, often called withdrawal, are the second real risk, and how often they occur is genuinely disputed:

• Lancet Psychiatry, 2024 (Henssler and colleagues, 79 studies, 21,002 patients): 31 percent had at least one symptom after stopping an antidepressant, against 17 percent after stopping a placebo. Allowing for that placebo effect, the authors estimated about 15 percent, or one in six to seven patients. Severe symptoms were uncommon: about 3 percent, against under 1 percent after placebo.
• Molecular Psychiatry, 2024 (Zhang and colleagues, 35 studies): about 43 percent, with no placebo adjustment. Rates rose with length of use, from about 35 percent after 6 to 12 weeks of treatment to 51 percent beyond 24 weeks.
• Psychological medicine, 2025 (Moncrieff and colleagues, a reanalysis of five studies that measured withdrawal systematically): 55 percent, with no placebo adjustment and wide uncertainty.
• JAMA Psychiatry, 2025 (Kalfas and colleagues, 50 randomized trials, 17,828 people): stopping added about one symptom on a standard checklist in the first week, with dizziness the most common.

These numbers differ mostly because the methods differ. Subtracting what people report after stopping a placebo shrinks the estimate. Most trials enrolled people who had taken the drug for weeks, not years, and longer use was linked to higher rates. The 2025 reanalysis argued that the 2024 Lancet Psychiatry review leaned on short industry-sponsored trials, counted side effects people happened to report rather than asking everyone systematically, and may have mistaken some withdrawal for relapse.

The camps from the serotonin argument reappear here. Moncrieff and Mark Horowitz co-wrote both the umbrella review and the withdrawal reanalysis, while Jauhar and Cowen are among the authors of the JAMA Psychiatry analysis. That does not tell you who is right, but it is a reason to read any single estimate alongside the others.

Two findings can look contradictory but are not. A 2019 review by Horowitz and David Taylor warned that withdrawal can be mistaken for the return of depression. The 2025 JAMA Psychiatry analysis found that, in trials, stopping was not associated with worsening mood scores, and its authors read later depression as relapse. One concerns how symptoms get labeled in a clinic; the other concerns average mood scores in trials.

The 2024 Molecular Psychiatry analysis linked higher risk to female sex, younger age, higher doses and longer use. It also listed abrupt stopping, although its own direct comparison of tapering with abrupt stopping found no statistically significant difference. The National Institute of Mental Health says people should not stop a prescribed medication without a provider’s help, because a provider can lower the dose slowly and safely. Whether to stop, how quickly, and how to tell withdrawal from relapse are decisions to work out with the prescriber.

NIMH also notes that children, teenagers and young adults under 25 may have more suicidal thoughts or behavior on antidepressants, especially early in treatment or after a dose change, and that people of every age taking them should be watched closely.

Questions to ask before buying a brain-chemistry test or product

1. Which organ made the chemical being measured? Urine dopamine is shaped by the kidney; blood serotonin mostly by the gut. Ask the seller how the test accounts for that.
2. Where is the validation? Look for published evidence that results predict a condition in an individual, not group averages from one small study.
3. Who profits from the result? When the company that runs the test also sells the products to “correct” it, weigh that conflict.
4. Is “boosts dopamine” or “supports serotonin” an outcome? Raising a chemical is a mechanism. Ask what people who used the product noticed, compared with people who did not.
5. What else do you take? If the product claims to act on serotonin and any prescription you take does too, the interaction matters more than the marketing.

When to get help today

• Possible serotonin syndrome: agitation, shivering, muscle spasms, fever, heavy sweating, a racing heart or confusion soon after starting, raising or combining serotonin-acting products. MedlinePlus says to contact a provider right away and notes that people with it are usually watched in a hospital for at least 24 hours. Severe symptoms are an emergency: call 911.
• Thoughts of suicide or self-harm: call or text 988, or chat at 988lifeline.org.
• New dizziness, nausea, mood changes or other symptoms around a dose change: whether they reflect withdrawal, a side effect or relapse is the prescriber’s question to help answer.
• St. John’s wort alongside any prescription, especially birth control, blood thinners, or transplant, HIV or cancer drugs: whether the combination is safe is for a pharmacist or prescriber to check.

The takeaway

Dopamine is better understood as a teacher than a reward, and serotonin is two separate systems, only one of which lives in your brain. With those two facts in hand, most brain-chemistry marketing becomes easy to read. A test or product that claims to measure or fix your brain’s dopamine or serotonin from a urine cup or a capsule is selling a picture of the brain that the biology does not support.

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