(Not everyone with an MTHFR mutation experiences symptoms. For a long time I didn't. Best guess is my body changed after a period of intense stress.)
I take L-Methylfolate 15mg, which bypasses my digestion and provides my body with the processed materials directly.
The brand name for this is Deplin, which requires a prescription. I just switched to over-the-counter L-Methylfolate - it remains to be seen if it's as effective.
Fun fact: once I started taking Deplin, I had to come off of all my traditional antidepressants. They were at max dosage and having no effect, but once my brain was flooded with neurotransmitters, all the antidepressant side effects kicked in, including (perversely) amplifying depression symptoms.
Having a concrete scientific explanation for the empirical result I was seeing was a great confidence booster, bolstered some hypotheses I had, and helped me identify next steps in my journey.
Over the counter should be identical. I only prefer the prescription because it’s regulated and less likely to be fake. :)