Does low serotonin cause depression or anxiety?

This idea is everywhere.  The thought that we will feel happier if we just increase our serotonin level is alluring.  How simple.  But this is not how brain and body biology actually work.

In fact, there is no reliable evidence that people with depression or anxiety have globally “low serotonin” in the brain. Decades of studies measuring serotonin, its metabolites, receptors, and transporters have failed to show a consistent deficiency. In fact, some findings point in opposite directions depending on brain region, receptor subtype, stress state, inflammation, sleep, and prior medication exposure.

Serotonin is not a mood chemical in isolation. It is a signaling system.   It is deeply involved in sleep–wake regulation, threat detection, gut motility, pain modulation, temperature control, and autonomic tone. In some circuits, increased serotonergic signaling dampens distress. In others, it increases agitation, insomnia, or vigilance.

The psychiatric drug class of SSRIs complicate the story further. They do increase synaptic serotonin acutely, but their delayed clinical effects appear to involve adaptive changes—receptor desensitization, downstream neurotransmitters, and neuroplasticity—rather than simple correction of any theoretical deficiency.

Our takeaway here is not that serotonin is irrelevant. It is that linear explanations fail. Depression and anxiety emerge from interacting systems: stress physiology, sleep disruption, inflammation, metabolism, neurodevelopment, genetics, and our lived experience. Serotonin participates in that web, but it is not a master dial.

Simple stories are comforting. Biology does not conform to simple explanations.