I have received megadose folic acid on prescription before to treat depression. It's safer than low dose antipsychotics but does the same job. I was on 10-15 mg a day - and yes, I mean milligrams. They came in 5 mg tablets. I probably still have some around, because I occasionally still need them when premenstrual.
The one danger with folic acid is that if you take any B-vitamin in great quantities, it can knock out your body's reserves of the other B-vitamins. The reasons for this aren't fully understood. As long as you take a good B-vitamin supplement to make sure you don't lose the other B-vitamins, it's basically fine unless you're pregnant (when all bets are off, and the recommended levels of folic acid differ from week to week).
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The one danger with folic acid is that if you take any B-vitamin in great quantities, it can knock out your body's reserves of the other B-vitamins. The reasons for this aren't fully understood. As long as you take a good B-vitamin supplement to make sure you don't lose the other B-vitamins, it's basically fine unless you're pregnant (when all bets are off, and the recommended levels of folic acid differ from week to week).