Any dietary or drug treatment with high doses of micronutrients may override the body's own control mechanisms; therefore, micronutrient therapies may be associated with potential side effects and toxicities. High-dosed micronutrients should not be used without medical supervision.
Vitamin B6 supplements at doses much larger than those needed to prevent deficiency (see Intake Recommendations) have been used in an attempt to treat a wide variety of conditions. In general, well-designed, randomized controlled studies have shown little evidence that large supplemental (‘pharmacologic’) doses of vitamin B6 are beneficial (20).
In early assessments, women experiencing side effects of oral birth control medication (‘contraceptives’) in high doses, such as vomiting and depression, were thought to be vitamin B6 deficient. A number of clinicians prescribed high doses (100–150 mg/day) of vitamin B6 to women in order to relieve these side effects.
A placebo-controlled study in women on the lower dose oral contraceptives, which are commonly prescribed today, found that doses up to 150 mg/day vitamin B6 (pyridoxine) had no benefit in preventing side effects, such as nausea, vomiting, dizziness, depression, and irritability (21). A more recent examination of the NHANES data showed that plasma PLP concentration were significantly reduced in women who were taking oral contraceptives and did not use dietary supplements (45, 46). Also reduced plasma PLP levels were found in another study in women taking oral contraceptives (46, 47).
The use of vitamin B6 to relieve the side effects of high-dose oral contraceptives led to the use of vitamin B6 in the treatment of premenstrual syndrome (PMS).
A more recent review of 25 studies suggested that supplemental vitamin B6 up to 100 mg/day may be of value to treat PMS; however, only limited conclusions could be drawn because most of the studies were of poor quality (23).
Because a key enzyme in the synthesis of neurotransmitters such as ‘serotonin’ is PLP-dependent, it has been suggested that vitamin B6 deficiency may lead to depression.
However, clinical trials have not provided convincing evidence that vitamin B6 supplementation is an effective treatment for depression (20, 24), though vitamin B6 may have therapeutic effects in premenopausal women (24). A case-control study of elderly with depression showed that depression was associated with lower vitamin B6 levels (44).
Presently, well-designed studies are needed to show whether there is any true benefit of treating depression with vitamin B6.
Vitamin B6 has been used to treat nausea and vomiting during pregnancy (‘morning sickness’). The results of two double-blind placebo-controlled trials that used 25 mg pyridoxine every eight hours for three days (25) or 10 mg pyridoxine every eight hours for five days (26) suggest that vitamin B6 may be beneficial in alleviating morning sickness. Each study found a slight but significant reduction in nausea or vomiting in pregnant women. A review of placebo-controlled trials on nausea during early pregnancy found vitamin B6 to be somewhat effective (27). A systematic review showed evidence that vitamin B6 was associated with improved mild symptoms (48).
It should be noted that morning sickness also resolves without any treatment, making it difficult to perform well-controlled trials.
Several early studies by the same investigator suggested that vitamin B6 status was low in individuals with carpal tunnel syndrome and that supplementation with 100–200 mg/day over several months was beneficial (28, 29). A study in men not taking vitamin supplements found that decreased blood levels of PLP were associated with symptoms of carpal tunnel syndrome (30).
While a few trials have noted some symptomatic relief with vitamin B6 supplementation, double-blind placebo-controlled trials have not generally found vitamin B6 to be effective in treating carpal tunnel syndrome (20, 31).
Authored by Dr Peter Engel in 2010, reviewed and revised by Angelika Friedel on 14.06.2017