Prescribed 1 mg folic acid, take multi-vitamin with 400mg folic acid, plus eat cereal (400mcg folic acid). Is this too much? Why psychiatrist presc?

Co factor. Folic acid, particular some forms which cross the blood-brain barrier, have been show to enhance the effect of antidepressants. It isn't typically toxic because it is water-soluble so you pass out any extra. Deplin is a prescribed form of this, a version of folic acid which gets across the blood-brain barrier.