B vitamin vs vitamin B

B vitamin and vitamin B both are nouns.

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

As nouns, B vitamin and vitamin B are synonyms defined as:

B vitamin and vitamin B: originally thought to be a single vitamin but now separated into several B vitamins

Other synonyms of B vitamin include B-complex vitamin, B, B complex, vitamin B complex.

B vitamin (noun)	vitamin B (noun)
originally thought to be a single vitamin but now separated into several B vitamins	originally thought to be a single vitamin but now separated into several B vitamins