"Our comprehensive review of the major strands of research on serotonin shows there is no convincing evidence that depression is associated with, or caused by, lower serotonin concentrations or activity."

https://www.nature.com/articles/s41380-022-01661-0

"The chemical imbalance theory of depression is still put forward by professionals, and the serotonin theory, in particular, has formed the basis of a considerable research effort over the last few decades. The general public widely believes that depression has been convincingly demonstrated to be the result of serotonin or other chemical abnormalities, and this belief shapes how people understand their moods, leading to a pessimistic outlook on the outcome of depression and negative expectancies about the possibility of self-regulation of mood. The idea that depression is the result of a chemical imbalance also influences decisions about whether to take or continue antidepressant medication and may discourage people from discontinuing treatment, potentially leading to lifelong dependence on these drugs."

"We suggest it is time to acknowledge that the serotonin theory of depression is not empirically substantiated."