What is actually known about how these drugs work
Not much, honestly. An SSRI raises available serotonin within hours; people do not feel better for weeks. The two leading explanations for that gap are both unsettled, and the field says so in print.
We have not finished checking this source. No judgement either way. how we score evidence
Caveat on this rating: This row asserts an absence, which is the claim most easily overstated. What is established is that the delayed onset is not explained by the immediate neurochemical change, and that the field's own reviews present competing accounts rather than a settled one. What is NOT established is that either account here is correct: the emotional-processing model and the neuroplasticity model are both active research programmes with supporting and non-supporting findings, and this row deliberately reports that they are unresolved instead of ranking them. The claim that no clinical test measures a chemical imbalance is a statement about available assays, not a claim that no biological contribution exists. A narrative review is also a lower grade of evidence than a systematic one, and it is cited here for what the field says about its own uncertainty rather than as a pooled result.
The honest answer to "how do these drugs work" is that nobody knows. The people best placed to know say so in print, and that is worth more than either story.
Significant findings
A 2017 review in The Lancet Psychiatry by Harmer, Duman and Cowen opens by naming the problem with the simple account: "a purely neurotransmitter-based explanation for antidepressant drug action is challenged by the delayed clinical onset of most agents and the need to explain how neurochemical changes reverse the many different symptoms of depression."
That is the gap the serotonin sentence never covered. An SSRI blocks serotonin reuptake within hours of the first dose. People do not feel better for weeks. Whatever is doing the work, it is not the immediate chemical change.
The review sets out two live accounts and declines to choose between them. One is that the drugs alter emotional processing early - how the brain reads faces, words, threat - and that mood follows from weeks of living with a changed appraisal. The other is neural plasticity: the drug changes the brain's capacity to reorganise, and what happens next depends on what the person's life supplies. The authors treat these as possibly complementary rather than rival, and say the next job is working out which patients each applies to.
That is the state of it, and the negative space matters as much as the content. There is no test a clinician can order that measures a chemical imbalance. There is no threshold below which a person's serotonin counts as low. There is no settled account of why a drug acting in hours takes weeks to help. Lithium has been in psychiatric use since 1949 and its mechanism is still argued over.
Worth asking
An explanation ending in "we do not know yet" is not weaker than one ending in a molecule. It is more accurate, and it is a better basis for a decision than a sentence that was easier to say. So the question worth carrying out of this trail is not "what is my imbalance" - there is no such measurement - but the one a prescriber can actually answer: what are we watching to tell whether this is working, over what period, and at what point would we decide it is not?
Source
How do antidepressants work? New perspectives for refining future treatment approaches — Harmer CJ, Duman RS, Cowen PJ (2017)
Read the source: https://doi.org/10.1016/S2215-0366(17)30015-9
DOI: 10.1016/S2215-0366(17)30015-9
How this was scored
- Study design
- not recorded
- Funding
- not recorded
- Published in
- not recorded
- Sample size
- not recorded
- Preregistered
- not recorded
- Conflicts disclosed
- not recorded
- Independent of proponent
- not recorded
- Retracted
- No
We have not finished checking this source, so there is no scoring to show yet. “We have not checked this yet” and “this is disputed” are different statements, so no scored band is shown rather than a low one.
Read the full scoring rubric, including what it can't tell you.
Published September 10, 2026.
Questions
How strong is the evidence behind this?
veisund rates this source "not yet assessed". We have not finished checking this source. No judgement either way. One caveat travels with that badge: This row asserts an absence, which is the claim most easily overstated. What is established is that the delayed onset is not explained by the immediate neurochemical change, and that the field's own reviews present competing accounts rather than a settled one. What is NOT established is that either account here is correct: the emotional-processing model and the neuroplasticity model are both active research programmes with supporting and non-supporting findings, and this row deliberately reports that they are unresolved instead of ranking them. The claim that no clinical test measures a chemical imbalance is a statement about available assays, not a claim that no biological contribution exists. A narrative review is also a lower grade of evidence than a systematic one, and it is cited here for what the field says about its own uncertainty rather than as a pooled result. The score is calculated from recorded facts about the source — study design, funding, publication venue, sample size, preregistration — not typed in by an editor.
What is the source for this?
How do antidepressants work? New perspectives for refining future treatment approaches — Harmer CJ, Duman RS, Cowen PJ (2017). DOI: 10.1016/S2215-0366(17)30015-9. The full source is linked on this page so you can read it yourself.
Is this medical advice?
This is information to bring to your prescriber, not medical advice and not a reason to change anything on your own. Nothing here is an instruction to stop or reduce a medication. If you are in crisis, call or text 988.
This is information to bring to your prescriber, not medical advice and not a reason to change anything on your own. Nothing here is an instruction to stop or reduce a medication. If you are in crisis, call or text 988.