Vitamin B
The B vitamins are eight water-soluble vitamins that play important roles in cell metabolism. Historically, the B vitamins were once thought to be a single vitamin, referred to as Vitamin B (much like how people refer to Vitamin C or Vitamin D). Later research showed that they are chemically distinct vitamins that often coexist in the same foods.

Supplements containing all eight B vitamins are generally referred to as a vitamin B complex. Individual B vitamin supplements are referred to by the specific name of each vitamin (e.g. B1, B2, B3).

B vitamins deficiency
Several named vitamin deficiency diseases may result from the lack of sufficient B-vitamins.

Vitamin B1 (Thiamine) deficiency causes Beriberi. Symptoms of this disease of the nervous system include weight loss, emotional disturbances, Wernicke's encephalopathy (impaired sensory perception), weakness and pain in the limbs, periods of irregular heartbeat, and edema (swelling of bodily tissues). Heart failure and death may occur in advanced cases. Chronic thiamine deficiency can also cause Korsakoff's syndrome, an irreversible psychosis characterized by amnesia and confabulation.

Vitamin B2 (Riboflavin) deficiency causes Ariboflavinosis. Symptoms may include cheilosis (cracks in the lips), high sensitivity to sunlight, angular cheilitis, glossitis (inflammation of the tongue), seborrheic dermatitis or pseudo-syphilis (particularly affecting the scrotum or labia majora and the mouth), pharyngitis, hyperemia, and edema of the pharyngeal and oral mucosa.

Vitamin B3 (Niacin) deficiency, along with a deficiency of tryptophan causes Pellagra. Symptoms include aggression, dermatitis, insomnia, weakness, mental confusion, and diarrhea. In advanced cases, pellagra may lead to dementia and death.

Deficiencies of other B vitamins result in symptoms that are not part of a named deficiency disease.

Vitamin B5 (Pantothenic acid) deficiency can result in acne and Paresthesia, although it is uncommon.

Vitamin B6 (Pyridoxine) deficiency may lead to anemia, depression, dermatitis, high blood pressure (hypertension), water retention, and elevated levels of homocysteine.

Vitamin B7 deficiency does not typically cause symptoms in adults but may lead to impaired growth and neurological disorders in infants.

Vitamin B9 (Folic acid) deficiency results in elevated levels of homocysteine. Deficiency in pregnant women can lead to birth defects. Supplementation is often recommended during pregnancy. Researchers have shown that folic acid might also slow the insidious effects of age on the brain.

Vitamin B12 (Cobalamin) deficiency causes pernicious anemia, memory loss and other cognitive decline. It is most likely to occur among elderly people as absorption through the gut declines with age. In extreme (fortunately rare) cases, paralysis can result.