Acetaminophen (a.k.a) relieves some types of headache. But this does not mean that these headaches are caused by acetaminophen deficiency. The brain doesn’t even produce acetaminophen.
The point of this analogy is to make clear that a medicine can work even if it is not acting on the cause of the symptom. In many cases a medicine can work even when the cause of the symptom is completely unknown.
This is the case with SSRIs — they alleviate some forms of depression, and also fail to work for many depressed people, but we still don’t know why. Lots of active research is going into this.
Quite a bit of research suggests that serotonin affects , so it seems as if the effectiveness of SSRIs arise through changes to neuronal circuits, rather than through changing the levels of serotonin as such. This would explain why the beneficial effects of SSRIs typically take weeks to show up, even though the levels of serotonin rise on the order of hours after taking a tablet. Changes to neuronal circuits involved in emotion and motivation may take quite a while to become effective.
Anyway, SSRIs remain controversial, and not just because they often fail to work. Even in people for whom it does work, the effects may not be long-lasting, and can often come with. Much more research will be needed before we can say we understand what is going on with these medications, and with the disorders they are aimed at treating.
This originally appeared on November 14, 2018 as an answer to the following question on Quora: