Multivitamins and Megavitamins

Megavitamin companies claim that taking very high doses of vitamins with or without high doses of minerals will increase the amount of nutrients available to help the body stay healthy or achieve higher levels of health.

Proponents of multivitamins and megavitamin have identified several similar benefits of these products:

• Promote good health by giving the body micronutrients it needs

• Promote a healthy immune system

• Support eye health as well as healthy hair and skin

• Boost energy levels and feelings of well-being and decrease stress

• Treat diseases caused or exacerbated by deficiencies in certain vitamins and minerals

• Prevent or treat diseases that are more common in older adults, including diabetes, heart disease, high blood pressure, and declines in brain function.

Multivitamins can help people get recommended levels of nutrients when foods alone are not providing enough vitamins and minerals.

• Studies show that people who take multivitamins regularly are more likely to have adequate levels of needed vitamins and minerals than people who do not take these dietary supplements. But the risk of getting too much of certain vitamins and minerals is also higher in multivitamin users.

• Studies have also shown that multivitamin users tend to have healthier lifestyles and diets than people who do not take these supplements. For this reason, it is difficult to tell whether the prevention of chronic diseases and the promotion of good health in multivitamin users is due to their use of these supplements or because they tend to have healthier diets and lifestyles.

• Research shows that using multivitamins does not seem to reduce the overall risk of chronic disease, but several nutrients in these products might help some groups of people. For example, taking preparations containing calcium and vitamin D might reduce the risk of fracture in women who have gone through menopause.

• Among people in high-resource countries who consume a varied and healthful diet, vitamin deficiencies are rarely a problem, with only occasional exceptions (for example, vitamin D, sometimes vitamin B12); and most mineral deficiencies tend to develop only in people known to be at risk for them, such as people who take diuretics or have other medical disorders.

• Several large studies have found no clear evidence that vitamin supplements can prevent chronic diseases or death due to major chronic diseases in people who do not have any nutritional deficiencies.

Taking multivitamins that provide nutrients in amounts that are similar to recommended daily intakes should be safe for most healthy people. However, there are certain risks:

• Large doses of beta-carotene, vitamin E, and possibly vitamin A in multivitamins or megavitamins can increase the risk of death.

• Multivitamins and megavitamins containing large amounts of vitamin A or beta-carotene could increase the risk of lung cancer in people who smoke or who used to smoke.

• Use of multivitamins or megavitamins containing large amounts of vitamin A by pregnant women could increase the risk of birth defects in their babies.

• Use of excessively high doses of certain vitamins (for example, vitamin A) can cause toxicity.

• Large doses of vitamin D can elevate the level of calcium in the blood (seeHypercalcemia).

• Use of supplements with large amounts of calcium can increase the risk of a heart attack, and high doses of some antioxidants (such as vitamin E) could increase the risk of prostate cancer in men.

• If the multivitamin or megavitamin capsule or pill is very large, older people may have difficulty with swallowing and be at risk of choking.

Multivitamins that provide nutrients in amounts similar to recommended daily intakes do not usually interact with drugs, except that multivitamins containing vitamin K can decrease the effectiveness of medicines to reduce blood clotting (like warfarin) because vitamin K helps blood clot.

• Megavitamin supplements can interact with several medications, depending on their ingredients.

• A supplement containing vitamin C, vitamin E, beta-carotene, and selenium can block the benefits of the combination of simvastatin and niacin on cholesterol levels in some people with coronary heart disease.

• Vitamin E can thin the blood, so taking a megavitamin containing a large dose of vitamin E as well as the blood thinner warfarin or aspirin could increase the risk of internal bleeding or stroke.

• Absorption of oil-soluble vitamins (A, D, E, and K) and beta-carotene may be impaired if taken at the same time as orlistat, a weight-lowering medication.

• High-dose vitamin A may result in liver toxicity if a person habitually consumes large amounts of alcohol.

• Multivitamin or megavitamin products may sometimes contain biotin (used to promote healthy hair, skin, and nails), and large quantities may interfere with the accuracy of certain laboratory tests, such as those that evaluate thyroid function, vitamin D levels, or troponin (a laboratory test to detect these proteins can help determine whether a person has had a heart attack).

• A large dose of Vitamin C may reduce the effect of the blood thinner warfarin.

• Taking a large dose of vitamin C with oral contraceptives or hormone replacement therapy may increase estrogen levels.

• Taking a large dose of vitamin C can have effects on certain cancer chemotherapy drugs (for example, bortezomib, paclitaxel, doxorubicin), possibly changing their effectiveness.

Although multivitamins can help people get recommended levels of nutrients, it is unusual for people in high-resource countries not to get enough vitamins and minerals to meet their bodies' needs from foods alone. People should ask their doctor about risks for a vitamin or mineral deficiency.

Multivitamins and megavitamins are not recommended for people who do not have any nutrient deficiencies because the claimed health benefits have mostly not been confirmed in high-quality studies, and these benefits are often outweighed by the risks of taking large doses.

Multivitamins containing doses of vitamins and minerals that are similar to recommended daily intakes are probably safe for most people. But supplements, such as megavitamins, containing larger amounts than those recommended can have unwanted effects and can even be harmful.

• Pregnant women should avoid taking multivitamins or megavitamins containing vitamin A. (Conventional prenatal vitamin preparations are preferable.)

• People who currently smoke or have done so regularly in the past should avoid taking multivitamins or megavitamins containing large doses of vitamin A or beta-carotene.

• People who do not have a vitamin or mineral deficiency, those who take certain medications (including blood thinners and the combination of simvastatin and niacin), and probably all people should talk to their doctor before using multivitamins or megavitamins.

• People taking orlistat should discuss supplementation of fat-soluble vitamins (A, D, E, and K) and beta-carotene with their physician.

• Men may choose to avoid vitamin E supplements, which may help avoid an increased risk of prostate cancer.