Introduction
Vitamin B6, also known as pyridoxine, is a water-soluble vitamin that serves as an essential cofactor in numerous enzymatic reactions throughout the body. As one of the eight B-complex vitamins, it plays crucial roles in amino acid metabolism, neurotransmitter synthesis, hemoglobin formation, and glycogen metabolism. Unlike fat-soluble vitamins, vitamin B6 is not stored in significant quantities in the body and requires regular dietary intake or supplementation.
Mechanism of Action
Vitamin B6 functions primarily as a coenzyme in its active form, pyridoxal 5'-phosphate (PLP). PLP serves as a cofactor for over 100 enzymes involved in:
- Amino acid metabolism (transamination, decarboxylation, desulfuration)
- Neurotransmitter synthesis (serotonin, dopamine, GABA, norepinephrine)
- Hemoglobin synthesis and function
- Glycogen phosphorylase activity
- Homocysteine metabolism
- Tryptophan-to-niacin conversion
These enzymatic activities support protein metabolism, cognitive development, immune function, and red blood cell formation.
Indications
FDA-Approved Indications:- Treatment and prevention of vitamin B6 deficiency
- Management of certain inherited metabolic disorders (pyridoxine-dependent epilepsy, homocystinuria, pyridoxine-responsive anemia)
- Management of nausea and vomiting in pregnancy (NVP)
- Adjunctive therapy for premenstrual syndrome (PMS)
- Treatment of certain drug-induced neuropathies (isoniazid-induced neuropathy)
- Homocysteine reduction in cardiovascular risk management
- Management of certain seizure disorders in infants
Dosage and Administration
Standard Adult Dosing:- Dietary supplement: 1.3-1.7 mg daily
- Deficiency treatment: 10-50 mg daily
- Drug-induced neuropathy prophylaxis: 10-50 mg daily
- Nausea in pregnancy: 10-25 mg every 8 hours
- Renal impairment: No dosage adjustment typically needed
- Hepatic impairment: Use with caution; monitor closely
- Elderly: May require higher doses due to decreased absorption
- Pediatrics: 0.5-1.0 mg daily for maintenance; higher for deficiency
- Oral administration preferred
- May be taken with or without food
- Available as tablets, capsules, liquids, and injectable forms
Pharmacokinetics
Absorption: Readily absorbed from the jejunum via passive diffusion; bioavailability approximately 75% for oral doses Distribution: Widely distributed throughout body tissues; primarily in liver, muscle, and brain; crosses placenta and enters breast milk Metabolism: Converted to active form (PLP) in the liver; extensive interconversion between pyridoxine, pyridoxal, and pyridoxamine forms Elimination: Primarily renal excretion as metabolites; half-life approximately 15-20 days; minimal storage with daily requirement for replenishmentContraindications
- Hypersensitivity to vitamin B6 or any component of the formulation
- No absolute contraindications exist for appropriate therapeutic use
Warnings and Precautions
- Neurotoxicity: High doses (>500 mg/day) may cause sensory neuropathy
- Dependency: Prolonged high-dose use may lead to dependency
- Masking symptoms: May mask symptoms of other deficiencies
- Pregnancy: Use during pregnancy should be at recommended doses only
- Renal impairment: Monitor closely in severe renal dysfunction
Drug Interactions
Significant Interactions:- Levodopa: Vitamin B6 may decrease effectiveness by enhancing peripheral metabolism
- Isoniazid: Increases vitamin B6 requirements; concomitant supplementation recommended
- Penicillamine: May decrease vitamin B6 levels
- Phenytoin/Phenobarbital: May decrease serum levels of these anticonvulsants
- Oral contraceptives: May increase vitamin B6 requirements
Adverse Effects
Common (≥1%):- Nausea
- Headache
- Drowsiness
- Mild gastrointestinal discomfort
- Sensory neuropathy (with prolonged high-dose use >500 mg/day)
- Photosensitivity
- Ataxia
- Numbness and paresthesia
- Reduced serum folic acid levels
Monitoring Parameters
- Clinical signs of deficiency or toxicity
- Neurological examination (for high-dose therapy)
- Plasma PLP levels (if available)
- Homocysteine levels (if used for hyperhomocysteinemia)
- Complete blood count
- Liver function tests (with high-dose long-term use)
Patient Education
- Take as directed by healthcare provider; do not exceed recommended doses
- Inform healthcare provider of all medications being taken
- Report any numbness, tingling, or unusual sensations immediately
- Maintain balanced diet with natural sources of vitamin B6 (meat, fish, poultry, potatoes, bananas)
- Store supplements at room temperature away from moisture
- Seek medical attention if experiencing severe side effects
References
1. National Institutes of Health. Vitamin B6 Fact Sheet for Health Professionals. Office of Dietary Supplements. 2021. 2. McCormick DB. Vitamin B6. In: Ross AC, et al., editors. Modern Nutrition in Health and Disease. 11th ed. 2014. 3. Dalton K, Dalton MJ. Characteristics of pyridoxine overdose neuropathy syndrome. Acta Neurol Scand. 1987;76(1):8-11. 4. Simpson JL, et al. Vitamin supplementation for preventing miscarriage. Cochrane Database Syst Rev. 2019. 5. Lheureux P, et al. Pyridoxine in clinical toxicology: a review. Eur J Emerg Med. 2005;12(2):78-85. 6. EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on Dietary Reference Values for vitamin B6. EFSA Journal. 2016;14(6):4485. 7. American College of Obstetricians and Gynecologists. ACOG Practice Bulletin No. 189: Nausea and Vomiting of Pregnancy. Obstet Gynecol. 2018;131(1):e15-e30.