While it is not "mandatory" in the sense that the supplements won't work without it, Vitamin C is highly recommended as a co-factor
Determination of relative methionine bioavailability in lactating cows fed smartamine m, mepron, and aminoshure m using the plasma-free AA doseresponse method
While drugs on your plans formulary, or list of preferred medications, dont always require prior authorization, many drugs do need to be reviewed by your health insurance company for medical necessity before they cover them
Moreover, hippocampal neurogenesis reduces neuronal damage by replacing lost neurons, which appears to improve depressive-like behavior (Zhang et al., 2008)
Its more direct and more efficient than, say, getting your B12 through eating meats high in the vitamin