top of page

Serotonin Levels in Depression: Here’s What You Really Need to Know

Updated: Feb 9

✎ Written by: Dubravka Rebic ✓ Fact-checked by: Dr. Nathan Brown, Ph.D.

This year, people all around the world were astounded to hear about a recent discovery regarding the cause of depression. Headline after headline highlighted the findings of a scientific review that changed what 280 million people who suffer from depression worldwide believed to be true.

But why did this new discovery cause such a commotion?

Since the 1960s, we have been led to believe that depression is caused by biochemical imbalances in the brain, in particular by lower levels or reduced activity of serotonin (a neurotransmitter that plays a key role in regulating digestion, mood, sleep, and other bodily functions).

Based on this belief, doctors all around the world have been prescribing a type of antidepressant called a serotonin reuptake inhibitor (SSRI) in order to help their patients alleviate depression symptoms by increasing serotonin levels. As a result, millions of people started taking medication because they thought they had a chemical imbalance in their brain that required an antidepressant to put it right.

However, up until this point, there has been no comprehensive review of the research on the link between serotonin and depression. Not until this newly published umbrella review, which showed that we have no convincing evidence that depression is caused by lower levels or reduced activity of serotonin after all, and called into question what antidepressants actually do.

Antidepressants Work, But Scientists Are Not Sure How

Even though the study findings are groundbreaking, they don't negate the effectiveness of antidepressants for many people. In fact, the study tried to answer the question: "Do individuals with depression have lower levels of serotonin?" And, as the authors suggested, the answer is no, but the question about the role and efficacy of antidepressants wasn't addressed in the paper.

The major key findings from the review are:

  • There's no difference in the serotonin levels and their breakdown products in the blood or brain fluids between people with depression and those without.

  • Based on studies involving thousands of patients, there is no association between the gene for the serotonin transporter and depression. Similarly, no gene-stress interaction effects were found.

So, if there's no connection between low serotonin and depression, how do antidepressants work exactly?

One recent article argues that antidepressants typically take up to two weeks to impact the neurotransmitter system and to show clinical effects. By contrast, they may exert early effects by affecting neuroplasticity – altering how the brain functions over time – which translates into later clinically noticeable changes.

​Neuroplasticity is the capability of the brain to alter its structure or function in response to exposure to new stimuli or environments. The study mentioned above showed that SSRIs might foster neuroplasticity or cause stress to ha