Here is a dosage recommendation for supplementation with Folate/Folic acid during the treatment of depression. It’s the conclusion offered by Simon N. Young, Dept. of Psychiatry, McGill University in a 2004 review article:
What about the recommendation that 2 mg of folate be given during the acute, continuation and maintenance treatment of depression? The actual dosage may be debatable; 1 mg may suffice, particularly in countries where there is voluntary or compulsory fortification of food with folate, and the addition of a vitamin B12 supplement may be prudent, but the general principle is reasonable. With our current knowledge, the potential benefits seem to far outweigh any disadvantages.
Reference: Simon N. Young, “Folate and depression—a neglected problem” in J Psychiatry Neurosci. 2007 March; 32(2): 80–82.
For further discussion, see the NYBC entries: