Our new review of serotonin research collated research from six different areas. We looked at research on serotonin levels in body fluids, levels of the main metabolite (breakdown product) of serotonin in the cerebro-spinal fluid (brain fluid), serotonin receptors, the serotonin transporter protein (the protein that removes serotonin from the synapse where it is active- this is what SSRIs [selective serotonin reuptake inhibitors] inhibit), studies testing the effects of reducing serotonin levels in volunteers and studies of the gene for the serotonin transporter. We found no convincing evidence from any of these fields of research that depression is connected with low serotonin, let alone caused by it. The article itself is here and here is an article about it in The Conversation.

The publicity this article has received demonstrates just how many people have been persuaded that depression is caused by a serotonin imbalance. I realise this may be unsettling news to many people who decided to take antidepressants because they were told and believed this was true. Some people may wish to reconsider whether taking antidepressants is the best thing for them. Therefore I would like to warn everyone not to stop their treatment suddenly. Stopping antidepressants can precipitate withdrawal effects which can be severe for some people, especially if they have been taking antidepressants for a long time. Therefore I would advise people to discuss their treatment with their doctor and if they decide they want to try and stop it, to reduce it very gradually following the Royal College of Psychiatrists guidelines.