Which Vitamins Are Dangerous to Take at the Same Time?

Nutrition experts agree: A balanced diet provides all the necessary nutrients for most healthy people.

Some individuals should avoid certain combinations of dietary supplements. Credit: FotografiaBasica/iStock/GettyImages

However, the dietary supplement industry is booming. In fact, the Council for Responsible Nutrition estimates that 3 out of 4 Americans are regularly taking a vitamin or other dietary add-on.

If you're taking a supplement for any reason, you should know about a few combinations that can be dangerous for certain individuals.

Are There Vitamins You Shouldn't Take Together?

Your body needs adequate doses of vitamins A, B-complex, C, D, E and K every day for optimum health. Multivitamins typically include all of these 13 essential nutrients.

Unless specifically directed by a doctor, it's usually not a good idea to take individual vitamin or mineral supplements. Water-soluble vitamins, such as C and B-complex, can be flushed out of your system when there's an excess, but the fat-soluble vitamins — D, E, K and A — are stored in the body, so taking large amounts can be dangerous.

Read more: How to Determine if a Vitamin or Supplement Is Actually Right for You

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You don't need to worry too much about interactions between vitamins if you're taking a multivitamin and eating a fairly healthy diet. But when taking individual supplements, such as niacin or vitamin K, you may need to be cautious about potential interactions with other nutrients.

Calcium and Vitamin D Risks

Calcium and vitamin D are commonly recommended as tandem supplements for their beneficial role in bone health. The mineral calcium is also important for heart health.

But a July 2019 review published in the Annals of Internal Medicine indicates that there is a slight increase in the risk for stroke when these two supplements are taken together. This finding was based on data from the Women's Health Initiative (WHI), a long-term national health study that included more than 160,000 women.

There is some controversy around this conclusion, however. A 2013 article published in Current Atherosclerosis Reports indicated that the original findings of the WHI showed no association between the two supplements and stroke risk. And a 2017 article published in the Journal of the American Heart Association actually found that calcium supplements increase the risk of stroke, but combining them with vitamin D seemed to offset this result.

The takeaway? Have a conversation with your doctor about the risks associated with taking these two supplements together, to ensure the benefits outweigh the risks.

Niacin Combinations and Cholesterol

Taking a multivitamin that includes niacin, or vitamin B3, is typically safe in combination with other nutrients. However, if you take niacin supplements and medications to help control your cholesterol — specifically, simvastatin — there are a few combinations that you should avoid.

According to the MedlinePlus, which is powered by the U.S. National Library of Medicine, when you're taking a combination of niacin and simvastatin, supplementing with vitamins C and E or beta carotene could interfere with this duo's ability to raise your HDL (good) cholesterol.

A caveat: This interaction has only been studied in those who currently have coronary artery disease (CAD). It is not known if this interaction holds true if you do not have CAD.

Folate Supplements and B12 Deficiency

Vitamin B12 and folate are both commonly prescribed supplements, with the latter being especially important during pregnancy. Older adults, those following a vegan diet and individuals with gastrointestinal disorders are encouraged to take a B12 supplement. While taking folate and vitamin B12 at the same time is not problematic in general, higher folate levels can mask a B12 deficiency.

The problem? Low levels of vitamin B12 can lead to megaloblastic anemia, a condition in which the red blood cells are unusually large, but decreased in number. Taking large amounts of folate can correct the anemia, making you think that you are good to go. However, if the underlying B12 deficiency is not addressed and corrected, permanent nerve damage can result, according to the National Institutes of Health (NIH).

If you are at risk for a B12 deficiency, seek your doctor's advice regarding proper supplementation. And keep in mind that you should never consume more than 1,000 micrograms of folic acid from fortified foods or supplements unless directed by your doctor.

Vitamins K and E in Blood Clotting

Vitamin K plays a critical role in your body's ability to properly clot your blood. If your doctor recommends a vitamin K supplement, avoid taking high dosages of vitamin E, which may counteract the effects of vitamin K, according to MedlinePlus. In fact, vitamin E supplementation of over 800 IU can interfere with blood clotting and make your blood thinner.

This is particularly important to keep in mind for those taking blood thinning medications, such as warfarin. Taking this medication and vitamin E can increase your risk of bleeding. Always follow your doctor's advice about any dietary supplements if you take blood thinning medication.

Read more: Vitamin K: Why You Need It and Where to Find It

Calcium and Iron Absorption

Iron-deficiency anemia is the most common type of anemia and develops when you're not getting enough iron-rich foods, per the Mayo Clinic. Those diagnosed with this condition are typically advised to increase their dietary iron through foods, including beef and other meats, beans, lentils, dark green leafy vegetables and dried fruit. Your doctor may also suggest an iron supplement.

According to the NIH, taking calcium supplements may not allow your body to effectively absorb all the iron from your supplement, so it's recommended to take your iron and calcium supplements several hours apart to avoid this interaction.

What About Multivitamins?

Dietary supplements vary widely when it comes to the nutrients they provide, as well as the dosage of those nutrients. If you opt to take multivitamins, the Mayo Clinic recommends choosing one that provides no more than 100 percent of the daily value of each nutrient. If your doctor prescribes a dosage higher than the daily value of a particular nutrient, disclose any other vitamin and herbal supplements that you take so problematic combinations can be avoided.

Read more: Confused About Multivitamins? Here's How to Choose the Best One for You

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