It only helps me truly at 600mg, which I started taking accidentally, and that's double the UK max, which used to be 600mg but was reduced as there's a mildly higher statistical likelihood of seizures as side-effect with that dose. Afair USA max is 450mg.
If I take any less, the depression causing emotional dysregulation and intrusive/automatic thoughts (hypermentalisations) come back (n I'm an utter mess at baseline), though it's said/known that ND folk can more often have hypo or hyper reaction to certain chemicals.
I read it can have an amplifying affect on other drugs in one's system, so could be a causal part of serotonin syndrome if another medication is being taken, but I can't find that paper again, which in part regarded someone who started it for smoking cessation and found, apparently, that it worked in combination with some serotonin based med to lift them out of depression, though pinch of salt on the causality and reality of all that.
It's also for ADHD, but it doesn't (really) help my attention or memory, though my psychiatrist noted that having space in the mind from not having bad thoughts certainly helps better attention..