Vitamin B6 or pyridoxine, as part of the water-soluble vitamin B-complex family, is actually a mixture of 6 inter-convertible pyridine vitamers, or related compounds: pyridoxine, pyridoxamine, pyridoxal, and their 5′-phosphorylated forms (1). Although plants and microorganisms can synthesize B6 on their own, humans must acquire it from dietary sources. Because vitamin B6 is abundant in meats, fish, poultry, shellfish, leafy green vegetables, legumes, fruits, and whole grains, deficiencies are rare. For example, patients taking the antituberculosis drug isoniazide need to take vitamin B6 to prevent drug-induced peripheral neuropathy.
Patients may use supplemental B6 to treat symptoms related to heart disease, hypertension, peripheral neuropathies, carpal tunnel syndrome, and diabetes.
Preclinical studies show that systemic administration of certain B vitamins including B6 has neuroprotective, antihypertensive (2), antinociceptive (3), and antitumor effects (4). In humans, higher B6 intake may lower serum homocysteine concentrations (5), but did not reduce the risk of developing type 2 diabetes (6). High dietary intakes of folate and B6 have been associated with reduced risk of mortality from stroke, coronary heart disease, and heart failure (7). Vitamin B6 supplementation did not improve cognition in older adults (8) and was not more effective than placebo for carpal tunnel syndrome (9). There is also limited evidence of its effect on asthma susceptibility or established disease (10). Data on its use for premenstrual syndrome is mixed (11), and evidence is limited and weak for the prevention of pregnancy-related nausea (12) (13).
Some studies suggest a beneficial role for vitamin B6 in preventing colorectal cancer (14) (15) (16), and lower intake of dietary vitamin B6 is associated with increased risk of gastric adenocarcinoma (17) and pancreatic cancer (18). However, conclusions from the Women's Health Study indicate that supplementation with vitamins B6, B12, and folate did not decrease breast cancer risk (19), nor did it have an effect on overall risk of invasive cancer or breast cancer (20) (21). Vitamin B6 administration is ineffective in preventing chemotherapy-associated hand-foot syndrome (HFS; also known as palmar-plantar erythrodysesthesia [PPE]) (22) (23) (24) (25), with limited evidence suggesting that higher doses might be needed (26). In one study, it did reduce the need for capecitabine dose modifications and incidence of severe HFS, but did not positively impact chemotherapy effects (27). A multi-arm trial to evaluate therapies including high-dose B6 for peripheral neuropathy did not find it more effective than placebo (28).
High B6 intakes can have toxic effects including sensory and motor neuropathies, and some case reports of neurotoxicity have not been reversible (29). B6 levels can also be elevated due to environmental exposures or genetic defects (1).