The vitamin B-complex refers to all of the known essential water-soluble vitamins except for vitamin C. These include thiamine (vitamin B1), riboflavin (vitamin B2), niacin (vitamin B3), pantothenic acid (vitamin B5), pyridoxine (vitamin B6), biotin, folic acid and the cobalamins (vitamin B12).

"Vitamin B" was once thought to be a single nutrient . Researchers later discovered these extracts contained several vitamins, which were given distinguishing numbers, leading many people to the erroneous conclusion that these vitamins have a special relationship to each other. Further adding to confusion has been the "unofficial" designation of other, non-essential vitamins, as members of the B-complex, such as choline, inositol, and para-aminobenzoic acid (PABA).

Each member of the B-complex has a unique structure and performs unique functions in the human body. Vitamins B1, B2, B3, and biotin participate in different aspects of energy production, vitamin B6 is essential for amino acid metabolism, and vitamin B12 and folic acid facilitate steps required for cell division.

Each of these vitamins has many additional functions, though none that require all B-complex vitamins simultaneously. Human requirements for each B vitamin vary considerably—from 3 mcg per day for vitamin B12 to 18 mg per day for vitamin B3 in adult males, for example. So, taking equal amounts of each one—as provided in many B-complex supplements—makes little sense. Megadoses of B-complex vitamins sometimes taken to combat everyday stress, boost energy, or control food cravings, do not appear to offer benefit unless a person is deficient in one or more of them.

Most multivitamin-mineral products contain the B-complex along with the rest of the essential vitamins and minerals. Since they are more complete than B-complex vitamins alone, multiple vitamin-mineral supplements are recommended to improve overall micronutrient intake and prevent deficiencies.