Vitamin B6 (pyridoxine) is a water-soluble vitamin absorbed in the jejunum that is present in many foods, including whole grains, vegetables, fish, and meats. Pyridoxine is involved in over 100 different enzyme reactions in the body. These include amino acid metabolism, heme and neurotransmitter synthesis, and the metabolism of glycogen, lipids, and steroids.

Pyridoxal phosphate (PLP) is the most biologically active form of pyridoxine in the body.

Isolated vitamin B6 deficiency is uncommon and usually occurs in conjunction with other vitamin deficiencies. Alcohol use disorder; malabsorption syndromes; eating disorders, which result in general undernutrition; or medications (eg, isoniazid, theophylline, penicillamine, oral contraceptives) predispose patients to the development of pyridoxine deficiency.

Drugs induce vitamin B6 deficiency in various ways. For example, isoniazid binds to PLP, resulting in increased excretion of the active form of B6. Penicillamine inhibits vitamin B6-dependent enzymes. Oral contraceptives play a role in the increased turnover of vitamin B6.

Manifestations of pyridoxine deficiency are often nonspecific but may include fatigue, nausea, anorexia, paresthesia, depression, glossitis, dermatitis, peripheral neuropathy, and microcytic anemia. Severe vitamin B6 deficiency can result in seizures and personality changes characterized by irritability and confusion.

In infants, B6 deficiency may lead to diarrhea, seizures (pyridoxine-dependent seizures), and anemia.