Vitamin B6 (pyridoxine) functions as pyridoxal-5-phosphate (P5P), a coenzyme in over 100 reactions including amino acid metabolism, neurotransmitter synthesis (serotonin, dopamine, GABA), haeme synthesis, and homocysteine metabolism.

High-dose supplementation (above 50-100 mg/day) paradoxically causes peripheral neuropathy, making B6 one of the few water-soluble vitamins with significant toxicity. Deficiency also causes neuropathy, elevated homocysteine, and mood disturbances.