The SSRI Lexapro [0] helped my girlfriend relapse on cocaine, before i met her. Cocaine is a much stronger anti-depressant than the SSRI's due to its effects as a Mono-Amine Oxidase Inhibitor [2] (1st generation anti-depressants).
Eventually the professionals figured out she is a poor methylator, and can't convert folic acid into Folate. Folate (Vitamin B9) is important for DNA synthesis. Food companies fortify with folic acid because it's more stable than other forms of Vitamin B9, even though we now know that some people are harmed by this fortification [1]. On the whole, fortification seems to be beneficial (fewer deformed babies), but poor methylators and old people seem to have negative side effects from getting their vitamin B9 from folic acid.
She said adding the L-Methyl-Folate was like flipping a switch between "depressed" and "normal". Her involuntary medical professionals didn't say, "hey sorry we made a mistake, you're not actually broken", and continue to ignore the actual causes of her condition.
[0] https://en.wikipedia.org/wiki/Escitalopram
[1] https://now.tufts.edu/articles/folic-acid-too-much-good-thin... / https://www.futurity.org/folic-acid-too-much-of-a-good-thing...
[2] https://en.wikipedia.org/wiki/Monoamine_oxidase_inhibitor
(edit - added bit about cocaine being an MAOI)