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.

Exactly this - SSRI's efficacy was established based on improvements in reports from depressed people and we formed a theory about the mechanism based on the interactions we understood. As we try to prove that theory out it turns out our theories don't hold - but people who are depressed still improve when on SSRIs! So we're still working on the mechanism (which we always knew was incomplete at best) but this work isn't about the underlying efficacy of the drugs on the condition. It's about the nerdy explanation for why SSRIs work.

  but people who are depressed still improve when on SSRIs

Does that mean 'over 50%' of them improve, or is it, as is more common with pharmaceuticals, closer to a rounding error?

There was a deluge of media a couple decades ago about Prozac and its dramatic effects.

A few years later, I read some report that the studies testing its efficacy had ambiguous conclusions.

> Does that mean 'over 50%' of them improve, or is it, as is more common with pharmaceuticals, closer to a rounding error?

This question is more complicated than it appears.

One of the biggest challenges with depression studies is that the placebo group always improves dramatically, too. Using your terms, "over 50%" of the placebo group would likely show improvements in their depression inventories.

This makes it very complicated to interpret the studies, because now you have to look for how much more the active treatment group improves relative to the placebo group.

This is a huge detail that gets abused a lot by anti-pharma people, who write headlines about how SSRIs are "barely better than placebo" and then ignore the actual statistics. Another common tactic is to try to reframe the thresholds in different terms like "effect size" and then pool studies together to try to show that the "effect size" is below some arbitrary threshold.

Another challenge is that placebo response has been getting stronger over the years and nobody really knows why. Some antidepressant studies have even been halted because, ironically, the placebo group improved so much that there numerically wasn't much room left for the active group to be statistically better given the sample size. This is less of a problem with very large scale studies where smaller margins can be shown to be more statistically significant, but those are expensive and rare.

There isn't really a question about whether or not they are effective for many patients in the world of empirical treatment. However, if you go digging through the internet you can find plenty of commentary trying to convince you they don't work. Sadly, I've had some close friends and family members delay SSRI treatment for years because they read too many of these studies, but when they finally gave in and did a trial it turned their life around. The drugs aren't perfect and don't work for every situation, but they do work for a lot of people.