Vitamin B6, folic acid, and vitamin B12 all play a role in converting homocysteine to other substances within the body. By so doing, they consistently lower homocysteine levels in research trials,^1,2,3 a finding that is now well accepted. Several studies have used (and some doctors recommend) 400–1,000 mcg of folic acid per day, 10–50 mg of vitamin B6 per day, and 50–300 mcg of vitamin B12 per day.

Of these three vitamins, folic acid supplementation lowers homocysteine levels the most for the average person.^4,5 It also effectively lowers homocysteine in people on kidney dialysis.⁶ In 1996, the FDA required that all enriched flour, rice, pasta, cornmeal, and other grain products contain 140 mcg of folic acid per 3½ ounces.⁷ This level of fortification has led to a measurable decrease in homocysteine levels.⁸ However, even higher levels of food fortification with folic acid have been reported to be more effective in lowering homocysteine,⁹ suggesting that the FDA-mandated supplementation is inadequate to optimally protect people against high homocysteine levels. Therefore, people wishing to lower their homocysteine levels should continue to take folic acid supplements despite the FDA-mandated fortification program.

Deficiencies of iron, vitamin B12, and folic acid are the most common nutritional causes of anemia.¹⁰ Although rare, severe deficiencies of several other vitamins and minerals, including vitamin A,^11,12vitamin B2,¹³vitamin B6,^14,15vitamin C,¹⁶ and copper,^17,18 can also cause anemia by various mechanisms. Rare genetic disorders can cause anemias that may improve with large amounts of supplements such as vitamin B1.^19,20

In two double-blind trials, supplementation with vitamin B6 (10 or 25 mg three times per day) significantly reduced the severity of morning sickness.^21,22

Many,^{23,24,25,26,27} though not all,²⁸ clinical trials show that taking 50–400 mg of vitamin B6 per day for several months help relieve symptoms of PMS. A composite analysis of the best designed controlled trials shows that vitamin B6 is more than twice as likely to reduce symptoms of PMS as is placebo.²⁹ Many doctors suggest 100–400 mg per day for at least three months. However, intakes greater than 200 mg per day can cause side effects and should never be taken without the supervision of a healthcare professional.Vitamin B6 has also been reported to relieve cyclic breast pain.³⁰

Many,^{31,32,33,34,35} though not all,³⁶ clinical trials show that taking 50–400 mg of vitamin B6 per day for several months help relieve symptoms of PMS. A composite analysis of the best designed controlled trials shows that vitamin B6 is more than twice as likely to reduce symptoms of PMS as is placebo.³⁷ Many doctors suggest 100–400 mg per day for at least three months. However, intakes greater than 200 mg per day can cause side effects and should never be taken without the supervision of a healthcare professional.Vitamin B6 has also been reported to relieve cyclic breast pain.³⁸

Oral contraceptives can deplete the body of vitamin B6, a nutrient needed for maintenance of normal mental functioning. Double-blind research shows that women who are depressed and who have become depleted of vitamin B6 while taking oral contraceptives typically respond to vitamin B6 supplementation.³⁹ In one trial, 20 mg of vitamin B6 were taken twice per day. Some evidence suggests that people who are depressed—even when not taking the oral contraceptive—are still more likely to be B6 deficient than people who are not depressed.⁴⁰

Several clinical trials also indicate that vitamin B6 supplementation helps alleviate depression associated with premenstrual syndrome (PMS),⁴¹ although the research remains inconsistent.⁴² Many doctors suggest that women who have depression associated with PMS take 100–300 mg of vitamin B6 per day—a level of intake that requires supervision by a doctor.

Uncontrolled and double-blind research shows that vitamin B6 can be helpful for autistic children.^48,49,50 In these trials, children typically took between 3.5 mg and almost 100 mg of B6 for every 2.2 pounds of body weight, with some researchers recommending 30 mg per 2.2 pounds of body weight. Although toxicity was not reported, such amounts are widely considered to have potential toxicity that can damage the nervous system; these amounts should only be administered by a doctor. One prominent researcher has suggested that vitamin B6 is better supported by research than is drug treatment in dealing with autism.⁵¹