The Collaborative International Dictionary of English v.0.48:

vitamin M \vitamin M\ n.
   same as vitamin Bc; folic acid.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

vitamin M
    n 1: a B vitamin that is essential for cell growth and
         reproduction [syn: vitamin Bc, vitamin M, folate,
         folic acid, folacin, pteroylglutamic acid,
         pteroylmonoglutamic acid]