No single chemical causes depression. For decades, the popular explanation was that low serotonin levels were to blame, but a major 2022 review from University College London found no clear evidence that serotonin levels or serotonin activity are responsible for depression. The reality is that depression involves multiple chemical systems in the brain and body, and the old “chemical imbalance” story was always an oversimplification.
That doesn’t mean brain chemistry is irrelevant. Several chemical messengers, hormones, and biological processes interact to influence mood, and disruptions in any of them can contribute to depressive symptoms. Here’s what the science actually shows.
The Serotonin Theory and Why It Fell Apart
The idea that depression comes from low serotonin took hold in the 1960s and became one of the most widely repeated claims in mental health. It made intuitive sense: early antidepressants boosted serotonin and norepinephrine levels in the brain, and a drug called reserpine, which depleted these chemicals, produced sedation and sluggish behavior in animals. Researchers connected the dots and proposed that depression was essentially a shortage of these signaling molecules.
But when scientists tested this directly, the evidence didn’t hold up. Studies comparing serotonin levels and its breakdown products in the blood or brain fluid found no difference between people with depression and healthy participants. When researchers artificially lowered serotonin in hundreds of healthy volunteers by restricting the amino acid the body uses to make it, this didn’t trigger depression. Large genetic studies involving tens of thousands of people found no differences in the serotonin transporter gene between those with and without depression. One unexpected finding went even further: people who used antidepressants long-term actually had lower serotonin levels in their blood, suggesting these drugs may cause the brain to compensate by reducing serotonin production over time.
What these studies did find was that stressful life events had a strong, consistent effect on depression risk. The more adversity someone experienced, the more likely they were to become depressed. That pointed to something far more complex than a simple chemical shortage.
Chemicals That Do Play a Role
Even though no single molecule causes depression, several chemical systems are genuinely involved in mood regulation, and disruptions in them appear in many people with depression.
Serotonin, Norepinephrine, and Dopamine
These three signaling chemicals, collectively called monoamines, still matter. Serotonin helps regulate mood, sleep, and appetite. Norepinephrine influences alertness and energy. Dopamine drives motivation and the ability to feel pleasure. Most antidepressants work by increasing the availability of one or more of these chemicals at nerve connections in the brain. The key distinction is that low levels of these chemicals don’t straightforwardly cause depression the way low insulin causes high blood sugar. Instead, the systems that produce, release, and respond to them can become dysregulated in ways that contribute to depressive symptoms alongside many other factors.
Glutamate and GABA
The brain’s two most abundant signaling chemicals are glutamate, which excites nerve cells and makes them more likely to fire, and GABA, which calms them down. Together they maintain a balance between brain activation and inhibition. In people with major depression, researchers have found reduced levels of both, along with decreased expression of the receptors that detect them. This imbalance affects synaptic plasticity, the brain’s ability to strengthen or weaken connections between neurons, which is critical for learning, memory, and adapting to new experiences. The rapid antidepressant effects of certain newer treatments are thought to work partly by resetting glutamate signaling.
Growth Factors
Your brain constantly maintains and rewires its connections through proteins called neurotrophic factors. One of the most studied is BDNF (brain-derived neurotrophic factor), which supports the survival of existing neurons and encourages the growth of new ones, particularly in the hippocampus, a brain region involved in memory and emotional regulation. People with major depression consistently show lower blood levels of BDNF compared to healthy individuals. In animal studies, reducing BDNF impaired the birth of new brain cells and produced depressive behavior, while restoring it reversed those symptoms. Most antidepressants appear to increase BDNF over time, which may be one reason they take weeks to work.
Stress Hormones and the Cortisol Connection
Chronic stress is one of the strongest and most consistent risk factors for depression, and the chemical link runs through cortisol. When you’re stressed, your brain activates a hormonal cascade called the HPA axis, which ultimately floods the body with cortisol. In short bursts, this is useful. Under chronic stress, though, the system gets stuck in overdrive.
People with major depression frequently show overactivity of the HPA axis. Persistently elevated cortisol damages the hippocampus, shrinking it over time and impairing its ability to regulate emotions. It also suppresses the production of BDNF, reduces the birth of new neurons, and interferes with serotonin and dopamine signaling. So while cortisol itself doesn’t “cause” depression, chronic excess of it creates a chemical environment in the brain where depression becomes much more likely.
Inflammation and Immune Chemicals
One of the more surprising discoveries in depression research is the role of the immune system. People with major depression show elevated levels of inflammatory molecules called cytokines, particularly IL-6 (one of the most reliable blood markers found in depression), TNF-alpha, and IL-1. These aren’t just bystanders. Inflammatory cytokines cross into the brain and directly interfere with neurotransmitter production. For example, certain inflammatory signals reduce the availability of a chemical the brain needs to manufacture dopamine, which could help explain the loss of motivation and pleasure that characterizes depression.
Inflammation also activates the HPA axis, creating a feedback loop: stress raises cortisol, cortisol promotes inflammation, and inflammation further disrupts brain chemistry. This is why conditions involving chronic inflammation, such as autoimmune diseases or obesity, carry higher rates of depression.
Reproductive Hormones and Mood
Fluctuations in estrogen and progesterone explain why certain forms of depression cluster around reproductive events. During the menstrual cycle, pregnancy, and menopause, levels of these hormones shift dramatically, and they directly influence brain chemistry.
A particularly important player is allopregnanolone (ALLO), a byproduct of progesterone that enhances GABA activity in the brain, producing calming and anxiety-reducing effects. During pregnancy, ALLO levels increase roughly 40-fold. After delivery, they plummet, and this sudden chemical withdrawal appears to contribute to postpartum depression in some women. Imaging studies of women with postpartum depression show heightened activity of the enzyme that breaks down monoamines, along with reduced serotonin activity.
In premenstrual dysphoric disorder (PMDD), the issue isn’t abnormal hormone levels but an abnormal brain response to normal fluctuations. Over 50% of women with PMDD show alterations in a gene network that regulates how brain cells respond to estrogen and progesterone. Their brains are essentially more sensitive to the same hormonal shifts that other women experience without mood disruption.
The Gut’s Chemical Contribution
About 95% of your body’s serotonin is made in the gut, not the brain, and the trillions of bacteria living in your intestines produce or influence several mood-related chemicals. Gut bacteria generate GABA, serotonin, and short-chain fatty acids (SCFAs), which are molecules made when bacteria ferment dietary fiber. People with major depression tend to have depleted levels of SCFAs, and in animal studies, supplementing them produces antidepressant effects by reducing gut permeability and calming HPA axis reactivity.
The connection isn’t fully mapped yet, but the pathway is real: chemicals produced by gut microbes influence local gut signaling, get absorbed into the bloodstream, and modulate the function of distant organs, including the brain. This is why diet, antibiotics, and gut health have increasingly entered the conversation around depression.
Why “Chemical Imbalance” Is Misleading
The phrase “chemical imbalance” implies that depression works like diabetes: measure the chemical, find the shortage, replace it, problem solved. Depression doesn’t work that way. It involves overlapping disruptions in monoamines, stress hormones, inflammatory molecules, growth factors, gut-derived chemicals, and reproductive hormones, all shaped by genetics, life experience, and ongoing stress. Two people with identical depressive symptoms might have very different underlying biology.
This complexity is actually useful to understand, because it explains why no single treatment works for everyone and why approaches that address multiple systems, combining medication with therapy, exercise, stress reduction, and dietary changes, tend to produce better outcomes than any one intervention alone.