Yes, indeed, both B12 and folate are essential for mental health. In fact, major depression is negatively correlated with blood concentrations of both of these nutrients. In other words, depressives are significantly more likely to be deficient......
http://www.ncbi.nlm.nih.gov/pubmed/15671130

And, a folate deficiency can prevent an antidepressant from working....
http://www.ncbi.nlm.nih.gov/pubmed/10967371
http://www.ncbi.nlm.nih.gov/pubmed/15323594
(I had a superb reference, but I couldn't find it....lost everything in a computer crash).

Also, a U.S. government study showed a correlation between depression and low folate status in the general population. The British are currently conducting a large study of folate supplementation of antidepressant therapy.

I think it is malpractise that doctors don't *routinely* prescribe B-vitamins to their depressed patients.....and selenium, which also improves mood if deficient.

There are no adverse interactions to consider. None.

Lar