In this case report, the authors present two male vegetarians with depression who were not responding to an antidepressant regimen. The subjects were found to be deficient in vitamin B12, and subsequent supplementation with B12 resulted in effective responses to the same antidepressant regimen. Thus, the authors conclude, "Subjects with depression who do not respond to conventional antidepressants should be evaluated for nutritional factors."
The real issue is that patients should be evaluated for nutrient deficiencies prior to the use of medications. For example Vit B12, Vit. D etc
"Does B12 deficiency lead to lack of treatment response to conventional antidepressants?," Kate N, Agarwal M, et al, Psychiatry (Edgmont), 2010; 7(11): 42-4. (Address: Department of Psychiatry, Postgraduate Institute of Medical Education and Research, Chandigarh, India).