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Supplements You Shouldn't Overuse, Say Experts: Zinc, Iron and More

Too much of a good thing can be harmful to your health.
FACT CHECKED BY Emilia Paluszek

The supplement and vitamin market in the US has never been more profitable, being worth over $37 billion in 2022—but the FDA still classifies supplements as a subcategory of food, which can lead to safety issues. "This has huge consequences for the whole category of dietary supplements—from vitamins, minerals, probiotics and all sorts of new ingredients," says Pieter Cohen, MD, internist at Cambridge Health Alliance and associate professor of medicine at Harvard Medical School. "What it means is that the manufacturer can introduce anything into the market that they believe is safe. The FDA's job is to identify the products that are causing harm after they've been on the market and remove them from store shelves… Because the FDA isn't vetting these products before they show up on store shelves or on the internet, what happens is that they can pose unpredictable risks." 

Because of the way herbal supplements are packaged and marketed, it can be easy to forget that they can be dangerous. "Herbal supplements can cause real health damage," says Kevin Pho, MD. "In 2012, the FDA blamed them for causing over 50,000 adverse events annually. Some pills use fillers that are made up of rice, or worse, black walnut, which can severely affect those with nut allergies, while others contain unlabeled toxic ingredients. Many also interact with prescription drugs, like garlic and ginkgo biloba which can potentiate the effect of blood thinners and cause life-threatening bleeding."

Generally speaking, health experts recommend getting nutrients from food rather than supplements (unless of course your doctor recommends it for some sort of deficiency). "It doesn't make sense to me to take huge doses of vitamins and minerals unless there's a diagnosed problem, because there is so little evidence that they do good and sometimes a possibility that they might do harm," says Marion Nestle, MPH, PhD, professor of nutrition, food studies, and public health at New York University. "Pills are not a shortcut to better health and the prevention of chronic diseases," says Larry Appel, MD, director of the Johns Hopkins Welch Center for Prevention, Epidemiology and Clinical Research. "Other nutrition recommendations have much stronger evidence of benefits—eating a healthy diet, maintaining a healthy weight, and reducing the amount of saturated fat, trans fat, sodium and sugar you eat." Here are five supplements to never take a lot of, according to experts. Read on—and to ensure your health and the health of others, don't miss these Sure Signs You've Already Had COVID.



Vitamin pills spilling from an open bottle

Be cautious with zinc supplements, experts warn. "Are you supplementing with zinc? Be careful!" says Barbara Broggelwirth, RDN, CDN. "Excess zinc intake can lead to a copper deficiency which can cause neurological problems, including numbness and weakness in the arms and legs. Zinc is a trace mineral, meaning that we only need small amounts of it to support our health. It is found in cells throughout the body and supports the immune system to fight off pathogenic bacteria and viruses. The body also uses zinc to make proteins and DNA, helps wounds heal and is important for proper senses of taste and smell. Intakes of 150–450 mg of zinc per day have been associated with such chronic effects as low copper status, altered iron function and reduced immune function. Zinc can also interact with certain medications such as antibiotics, diuretics, and penicillamine (a drug used to treat rheumatoid arthritis)."

"Yes, too much zinc can be harmful," says the National Institutes of Health (NIH). "Signs of too much zinc include nausea, dizziness, headaches, upset stomach, vomiting, and loss of appetite. If you take too much zinc for a long time, you could have problems such as lower immunity, low levels of HDL ("good") cholesterol, and low copper levels. Taking very high doses of supplemental zinc can reduce your body's absorption of magnesium. Using large amounts of denture creams that contain zinc, well beyond what the label recommends, could lead to excessive zinc intake and copper deficiency. This can cause neurological problems, including loss of coordination, numbness, and weakness in the arms, legs, and feet."



assortment of red pills and capsules of iron supplements

Be careful about how much iron you take, experts say. "Iron is a fat-soluble nutrient," says NASM-certified personal trainer Maia Appleby. "When you take in more than you can use, your body excretes very little of it and stores the excess in your liver, tissues and other organs. If you overdose on iron supplements, seek immediate medical attention, as the damage to your organs can quickly elevate to serious levels. Because toxicity usually occurs due to excessive supplement intake, it is safer to get the iron you need from food. A 3-ounce serving of chicken liver, oysters or beef liver gives you 30 to 60 percent of your recommended daily intake for iron, while the same serving size of roast beef, dark turkey or ground beef provides 10 to 20 percent. Some plants, such as soybeans, lentils, black-eyed peas and spinach, are also good sources of iron, as are iron-fortified cereals and grains. Serve your iron-rich foods with a side of veggies — the vitamin C will enhance iron absorption."

"Iron can be harmful if you get too much," says the NIH. "In healthy people, taking high doses of iron supplements (especially on an empty stomach) can cause an upset stomach, constipation, nausea, abdominal pain, vomiting, and diarrhea. Large amounts of iron might also cause more serious effects, including inflammation of the stomach lining and ulcers. High doses of iron can also decrease zinc absorption. Extremely high doses of iron (in the hundreds or thousands of mg) can cause organ failure, coma, convulsions, and death. Child-proof packaging and warning labels on iron supplements have greatly reduced the number of accidental iron poisonings in children. Some people have an inherited condition called hemochromatosis that causes toxic levels of iron to build up in their bodies. Without medical treatment, people with hereditary hemochromatosis can develop serious problems such as liver cirrhosis, liver cancer, and heart disease. People with this disorder should avoid using iron supplements and vitamin C supplements."



Calcium supplement tablet pills on dark wooden background

Calcium is important, there is no question about that—but too much can be dangerous. "Calcium deficiency can, over time, lead to weak and brittle bones, which is called osteoporosis," says Dr. Donald Brown, primary care practitioner at Houston Methodist. "It's characterized by reduced bone density, increased bone loss and a higher risk of hip, wrist and spine fractures. Many people don't get enough calcium, actually. But the good news is that this can often be corrected with dietary changes, especially in those younger people who might be lacking. It's best for your calcium intake to come from your diet, which is very achievable since it's a mineral found in many foods. Those who follow a healthy diet are likely getting an optimal amount of calcium." 

So how much is the right amount? There is disagreement about the recommended dosage of 1000-1,200 mg being too high. "Regardless of whether 1,000 or 1,200 mg per day might be too high, meeting either requirement via your diet is usually still achievable," Dr. Brown says. "Just be sure you're eating a few servings of calcium-rich foods every day. There's also the potential for adverse effects. For instance, calcium supplements may lead to kidney stones since they cause more calcium to be eliminated via the urine. Additionally, these supplements might increase a person's risk of heart disease and prostate cancer, although the evidence is mixed and more research is needed."


Fat-Soluble Vitamins

woman taking vitamin D3

Fat soluble vitamins such as A, E, D, and K should be taken with caution. "Basically, there are two types of vitamins: water-soluble and fat-soluble," says Dawn Gerber, PharmD, a clinical ambulatory pharmacy specialist with Banner Health. "Although your body can excrete large amounts of water-soluble vitamins, it holds onto fat-soluble vitamins, which can be toxic at high levels." Fat-soluble vitamins are stored in the liver and fatty tissue. "These get caught in our fat, the non-muscular parts of our bodies," Dr. Gerber says. "The more supplements we take at high doses, the more that accumulates and isn't flushed out."

"Vitamin D toxicity, also called hypervitaminosis D, is a rare but potentially serious condition that occurs when you have excessive amounts of vitamin D in your body," says Katherine Zeratsky, RD, LD. "Vitamin D toxicity is usually caused by large doses of vitamin D supplements — not by diet or sun exposure. That's because your body regulates the amount of vitamin D produced by sun exposure, and even fortified foods don't contain large amounts of vitamin D. 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. Vitamin D toxicity might progress to bone pain and kidney problems, such as the formation of calcium stones."


Vitamin C

young woman taking pill
Shutterstock / New Africa

The daily limit for vitamin C should not be above 2000 mg a day for adults. "Taking too much vitamin C can cause diarrhea, nausea, and stomach cramps," says the NIH. "In people with a condition called hemochromatosis, which causes the body to store too much iron, high doses of vitamin C could worsen iron overload and damage body tissues. Vitamin C dietary supplements might interact with cancer treatments, such as chemotherapy and radiation therapy. It is not clear whether vitamin C might have the unwanted effect of protecting tumor cells from cancer treatments or whether it might help protect normal tissues from getting damaged. 

"If you are being treated for cancer, check with your healthcare provider before taking vitamin C or other antioxidant supplements, especially in high doses. In one study, vitamin C plus other antioxidants (such as vitamin E, selenium, and beta-carotene) reduced the heart-protective effects of two drugs taken in combination (a statin and niacin) to control blood-cholesterol levels. It is not known whether this interaction also occurs with other statins. Healthcare providers should monitor lipid levels in people taking both statins and antioxidant supplements."

Ferozan Mast
Ferozan Mast is a science, health and wellness writer with a passion for making science and research-backed information accessible to a general audience. Read more about Ferozan