vitamin M vs vitamin B complex

vitamin M and vitamin B complex both are nouns.

Word	Noun	Adjective	Verb	Adverb
vitamin M	Yes	No	No	No
vitamin B complex	Yes	No	No	No

As nouns, vitamin B complex is a hypernym of vitamin M; that is, vitamin B complex is a word with a broader meaning than vitamin M:

vitamin M: a B vitamin that is essential for cell growth and reproduction
vitamin B complex: originally thought to be a single vitamin but now separated into several B vitamins

Other hypernyms of vitamin M include B-complex vitamin, B, B complex, B vitamin, vitamin B.

vitamin M (noun)	vitamin B complex (noun)
a B vitamin that is essential for cell growth and reproduction	originally thought to be a single vitamin but now separated into several B vitamins