- The relationship between serum folate, vitamin B12, and homocysteine levels in major depressive disorder and the timing of improvement with fluoxetine. Papakostas, G.I., Petersen, T., Lebowitz, B.D., Mischoulon, D., Ryan, J.L., Nierenberg, A.A., Bottiglieri, T., Alpert, J.E., Rosenbaum, J.F., Fava, M. Int. J. Neuropsychopharmacol. (2005)