Vitamin and mineral deficiencies are an area of concern for the Centers for Disease Control and Prevention. The agency notes that such micronutrients are "vital to healthy development, disease prevention, and wellbeing," but that, with the exception of vitamin D, they are not produced in one's body and must be derived from dietary sources. 

While many people get the recommended amounts of nutrients from a healthy diet, some groups of people benefit from capsule or powder vitamin and mineral supplementation since some practices, conditions or behaviors limit the amount of nutrients one gets and "nutritional requirements change during life," explains Beth Czerwony, RD, a registered dietitian at Cleveland Clinic’s Center for Human Nutrition. These groups of people include vegans, pregnant women, the immunocompromised, the elderly, people with gastrointestinal disorders, people who smoke, drink or are taking certain medications, and individuals with food allergies or intolerances.

For most other people, however, micronutrient supplementation is often unnecessary and there are circumstances when it may even be harmful. 

Can you take too many vitamins?

As a general rule, not getting enough nutrients in one's diet is a broader area of concern than getting too many, which is why more research exists on that front. "We know far less about getting too much of essential nutrients than too little," says Alice Lichtenstein, DSc, director of the Cardiovascular Nutrition Team at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University. 

She adds that "findings continue to emerge," but that there is overall less concern about getting too many water-soluble vitamins than fat-soluble ones because excess amounts of water-soluble vitamins are usually urinated out of one's system whereas fat-soluble vitamins such as A, D, E and vitamin K are often absorbed. "We do not have the capacity to excrete excess amounts of fat-soluble vitamins so they can accumulate in the body and have adverse effects in some cases," Lichtenstein explains. 

What happens when you take too many vitamins?

Still, overdosing on "both water- and fat-soluble vitamins can cause a range of adverse reactions including irregular heartbeat, frequent urination, abdominal pain, weakness and appetite loss," cautions Alexandra Volo, DO, a family medicine physician at NYU Langone Medical Associates—West Palm Beach.

While any ingredient in a vitamin or mineral supplement can be toxic in large amounts, calcium and iron are the ones that carry the most serious risks. "People with severe iron overdoses may develop low blood pressure, liver failure, lung injury, coma and even death," warns Czerwony. 

She says that overdosing on calcium can "impair the functioning of the kidneys, increase the pH of the blood, and can cause nausea and vomiting, confusion, itching, and in extreme cases, irregular heartbeat." 

Additional overdose risks are associated with large quantities of niacin, vitamin C, magnesium, vitamin D and vitamin A. "The main consequence of vitamin D toxicity is a buildup of calcium in your blood (hypercalcemia), which can cause nausea and vomiting, weakness, and frequent urination," explains Czerwony. Heartburn and headaches are associated with overdosing on vitamin C, and excesses of vitamin A may cause drowsiness and irritability. 

Shawna Reigel, MD, a family medicine physician at Inspira Medical Group Primary Care Clarksboro, notes that the most common side effect of getting too much magnesium is diarrhea and that facial flushing is associated with getting "excess amounts of niacin." 

Is it OK to take many vitamins at once?

Beyond toxicity, another area of concern for some people is whether it's harmful to take many vitamins or minerals at the same time.  

While combining supplements usually won't interfere with their effectiveness, and, in some cases, may even be beneficial (such as how vitamin C helps with iron absorption), there are times when supplements may interact negatively with each other, so "it's important to know what supplements you are taking as many vitamin supplements contain several vitamins within a capsule, tablet, or powder and the chance for extra consumption and adverse effects are higher," says Jesse Bracamonte, DO, a family medicine physician at Mayo Clinic in Arizona.

It's also important to learn and adhere to the recommended dietary allowance (RDA) of any vitamin or mineral supplement one is taking. Such information can be obtained from one's physician, from the National Institutes of Health Office of Dietary Supplements or from academic organizations such as Harvard School of Public Health. 

"In general, it is not a good idea to take high doses of essential nutrients," advises says Lichtenstein. "If there is a concern about the adequacy of nutrient intakes, the best approach is to discuss with a healthcare provider. That would also avoid the risk of experiencing potential negative drug/nutrient interactions."

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