A 2022 review found no consistent evidence for a serotonin deficiency causing depression, and suggested long-term antidepressant use might reduce serotonin concentration.

“there recently was a interesting review which was published this is in molecular psychiatry a nature journal and it is from july 20th 2022 and this is a serotonin theory of depression systemic umbrella review of the evidence uh you can see here the conclusion they draw is that the main areas of serotonin research provide no consistent evidence of there being an association between serotonin and depression and no support for the hypothesis that depression is caused by lowered serotonin activity or concentrations some evidence was consistent with the possibility that long-term antidepressant use reduces serotonin concentration”