Why Your Antidepressant Makes You Wait
The chemistry is corrected on the very first day, yet nothing happens for weeks. That gap is the most interesting thing about these drugs, and it is the reason the explanation we were all given turned out to be wrong.
When I was a medical student, the explanation we were handed was short enough to deliver to a patient in under a minute. If you are depressed your serotonin is low, and a drug that raises it will make you feel better. I prescribed on that basis for years without ever examining it, and so did almost everyone I trained with.
I have now prescribed these medications for twenty years, mostly to people living with chronic pain, and it turns out to be considerably more complicated than that. The part we got wrong is not the part most people would guess.
The detail that never fitted
Consider what the drug actually does on the first day. Your brain cells do not touch one another; they are separated by narrow gaps, and chemical messengers ferry the signal across. A dedicated protein then retrieves whatever is left over and draws it back into the cell that released it. An SSRI blocks that retrieval, so more of the messenger lingers in the gap for longer.
All of that is complete within hours of the very first dose. If a shortage of serotonin were genuinely the problem, and the drug corrects that shortage the same afternoon, then relief ought to arrive the same afternoon. It does not, and it never has for anyone.
Every clinician who prescribes these drugs warns patients to expect several weeks of waiting,3 and we have done so for decades without ever having a satisfying reason for it. A hole that size in an explanation ought to bother the people repeating it.
So somebody went looking
In July 2022 a research team gathered every serious line of evidence connecting serotonin to depression and examined them side by side. They ran no new experiment of their own; they simply laid out what decades of accumulated work had actually produced.1
They examined serotonin metabolites, receptor binding, the transporter gene, and the studies that deliberately deplete serotonin in healthy volunteers to see whether mood follows it downward. Across six separate domains they found no consistent association between serotonin and depression, and no support for the idea that the illness is caused by having too little of it. Much of the coverage translated this into antidepressants do not work, which is emphatically not what the review said, and the distinction matters enormously if you are currently taking one.
| Year | Study | Finding |
|---|---|---|
| 2009 | Harmer, Goodwin & Cowen Br J Psychiatry | SSRIs shift the brain’s negative emotional bias within hours. The felt change in mood arrives weeks later. |
| 2018 | Cipriani et al. The Lancet | All 21 antidepressants studied outperformed placebo across 522 trials and roughly 116,000 patients. |
| 2021 | Casarotto et al. Cell | Antidepressants bind directly to TrkB, the receptor for BDNF, and drive plasticity. Serotonin is not required for the binding. |
| 2022 | Moncrieff et al. Molecular Psychiatry | No consistent evidence that depression is caused by low serotonin, across every major line of evidence. |
These do not disagree with each other. One asks how the drugs work. The other asks whether they work at all.
Rehab, not a painkiller
A painkiller acts quickly because it does nothing more than turn a signal down. Rehabilitation takes weeks because tissue has to physically change. Your antidepressant belongs firmly in the second category, and that single distinction explains almost everything about the way it behaves.
In 2021 a group working in Helsinki showed that these drugs bind directly to TrkB, the receptor for brain derived neurotrophic factor, which is the molecule that allows neurons to form, strengthen and rearrange their connections.2 Crucially, that binding occurs at brain concentrations reached only after sustained dosing rather than after a single tablet.
The useful effect, in other words, may have very little to do with serotonin. It may be plasticity: the brain recovering its capacity to rewire circuits that depression has worn into a groove. Building and pruning connections is slow structural work that takes weeks, which is precisely how long these drugs take. The wait was never a flaw in the medicine; the wait is the medicine working.
- The drug blocks the serotonin transporter Within hours of the very first dose. This part was never in doubt.
- Serotonin lingers in the synapse The entire chemical correction the old theory described is now complete. The patient feels no different.
- The drug binds TrkB directly The receptor for BDNF. Shown in Cell, 2021, at the same concentrations that work in patients. Serotonin is not required for this step.
- A window of plasticity opens Neurons become able to form, strengthen and prune connections again. Slow, structural work.
- The circuit reorganises, and mood follows Two to six weeks. Which is precisely the delay the serotonin story could never account for.
What follows, and what does not
“Depression is a chemical imbalance.”
Unproven
It was a convenient shorthand that hardened into a slogan, and it was never actually demonstrated.
“So antidepressants do not work.”
False
Being unable to explain why something works is an entirely different claim from it not working. The largest review we have pooled 522 trials and found that all of them outperformed placebo.4
“Then I should come off mine.”
Not from this
Nothing here is a reason to stop a medication, and stopping abruptly carries real risks of its own. That decision belongs with the person who prescribed it.
We were wrong about the mechanism and we were not wrong about the medicine. Those are very different failures, and only one of them should change what anyone does tomorrow.
References
- Moncrieff J, Cooper RE, Stockmann T, et al. The serotonin theory of depression: a systematic umbrella review of the evidence. Molecular Psychiatry, 2022
- Casarotto PC, Girych M, Fred SM, et al. Antidepressant drugs act by directly binding to TRKB neurotrophin receptors. Cell, 2021
- Harmer CJ, Goodwin GM, Cowen PJ. Why do antidepressants take so long to work? A cognitive neuropsychological model of antidepressant drug action. British Journal of Psychiatry, 2009
- Cipriani A, Furukawa TA, Salanti G, et al. Comparative efficacy and acceptability of 21 antidepressant drugs. The Lancet, 2018
Figure. Synapse and re-uptake diagram by Sabar, released into the public domain, via Wikimedia Commons.