We used to think that B vitamins were harmless because, like vitamin C, they’re water-soluble and therefore can’t accumulate in the body like fat-soluble vitamins (A, D, E, and K). However, we now know that too high an intake of certain B vitamins can be troublesome. For example, vitamin B6 (pyridoxine) can cause nerve toxicity although it usually doesn’t do that in doses lower than 200-300 mg per day. Over time, higher doses of B6 can be harmful and may result in nerve damage, producing numbness and tingling in the extremities that may eventually be irreversible. Be sure to discontinue use of supplemental B6 if you notice any unusual sensations. (I saw one woman who developed numbness in her legs from a daily dose of just 200 mg. A neurologist told her she might have multiple sclerosis, but when she stopped taking the B6, the symptoms disappeared.) I recommend a trial of 100 to 200 mg per day of this vitamin for nerve compression injuries such as carpal tunnel syndrome, but I always caution patients to discontinue it if they notice any unusual numbness. Too much B6 can also cause oversensitivity to sunlight, which can lead to skin rashes and numbness, as well as nausea, vomiting, abdominal pain, loss of appetite, and abnormal liver function.
High doses of niacin (vitamin B3) can also cause problems, but you’re unlikely to run into trouble unless you’re taking more than 2,000 to 3,000 mg per day in an effort to lower cholesterol. Reactions range from flushing, itching, nervousness and headaches to intestinal cramps. Don’t take doses in excess of 3,000 mg a day except under careful medical supervision. At those amounts nausea, jaundice and elevated liver enzymes can occur, a toxic picture mimicking hepatitis. The symptoms disappear when niacin is discontinued. Don’t take high doses of niacin if you’re pregnant, have ulcers, gout, diabetes, gallbladder disease, liver disease, or have had a recent heart attack. Anyone who takes this B vitamin to lower cholesterol should do so only under a physician’s supervision and should have liver function tests before the start of therapy and periodically thereafter. You also should monitor your cholesterol monthly and keep your niacin dose to the lowest possible level to maintain improvement.
The healthiest way to insure that you’re getting all the B vitamins and other nutrients you need is to eat a varied diet that includes fruit, vegetables, whole grains, legumes, healthy fats, and protein.
Andrew Weil, M.D.