The serotonin theory of depression: a systematic review of the evidence (2022)

Important to note that the serotonin theory of depression doesn't have to be strictly true for SSRIs to be effective. People who having passing familiarity with neuroscience often assume that psychiatric medications work by correcting deficiencies, but this isn't true. It's also not accurate to say that SSRIs "give you more serotonin" or any of the other variations on that theme.

Neurotransmitters aren't simple levels in the brain that go up and down, despite how much podcasters and fitness influencers talk about them like that. Neurotransmitter dynamics are complex and the long-term adaptations after taking medications like an SSRI can't be simply described in terms of "levels" going up and down. There are changes in frequency, duration, and movement of Serotonin across synapses that are much more complex. There are also adaptations to the receptors, including auto-receptors which modulate release of neurotransmitters (side note: some newer antidepressants also directly target those autoreceptors with possibly slight improvements in side effect profile).

So keep that in mind when reading anything about the serotonin theory of depression. This is often brought up as a strawman argument to attack SSRIs, but we've known for decades that the serotonin theory of depression never fully explained the situation. We've also known that some conditions like anxiety disorders are associated with increased serotonin activity in parts of the brain, which SSRIs can normalize.