Walk down the supplement aisle and it's easy to assume that if a little bit of a vitamin or mineral is good for you, more must be even better. But according to physicians who regularly field questions about supplement safety, that assumption can backfire—sometimes seriously. Several of the most popular vitamins, minerals, and herbal supplements on store shelves carry real risks when taken in high doses, and many people don't realize they may be getting the same ingredient from multiple sources at once, a phenomenon known as “stacking.”
“If you're deficient, it makes sense to replace it responsibly, potentially under the supervision of a healthcare worker,” said an orthopedic surgeon who regularly discusses supplement safety with patients. “But sometimes people think, well, if this is good, then 10 times more is better—and you need to be aware of what could happen.” His colleague, a functional medicine physician, agreed, noting that most people don't need to take a supplement at all unless they're deficient in something confirmed by a blood test, following a restricted diet, or becoming more frail with age.
Here are 10 commonly used supplements that physicians say deserve a second look, along with the risks that come with overdoing them.
1. Vitamin B6
Vitamin B6, also known as pyridoxine, is a water-soluble vitamin, which leads many people to assume any excess simply gets flushed out of the body. That's only partly true. “Your body can pee it out, but it can't necessarily pee it out at a rate that's acceptable,” the orthopedic surgeon explained. Circulating excess B6 can damage nerves, a condition that is neurotoxic in high doses.
The effects can include numbness, tingling, and other vague neurological symptoms that are often not immediately linked back to the supplement. Concerning still, if someone stops taking excessive B6, symptoms can take months to improve—and sometimes don't fully resolve, leaving lasting nerve damage. Blood testing can confirm B6 levels if excess intake is suspected.
2. Vitamin D
Unlike B6, vitamin D is fat-soluble, meaning any excess is stored in body fat rather than excreted. Vitamin D has received significant attention over the past decade for its role in immune health and mood, but taking mega-doses—think 10,000 to 20,000 international units—without medical supervision can lead to toxicity.
“What I would say is that if this is prescribed by a physician, sometimes this is indicated, and supervised, it's okay,” the functional medicine physician said. Unsupervised use above roughly 4,000 international units carries a higher risk of toxicity, which is tied to disrupted calcium metabolism. The resulting hypercalcemia, or excess calcium in the bloodstream, can cause nausea, vomiting, constipation, muscle aching, and even heart arrhythmias. Physicians note that true vitamin D toxicity remains rare, but is worth being aware of.
3. Turmeric
Turmeric has become a pop-culture darling, showing up in everything from lattes to capsules marketed for inflammation. Using it as a cooking spice isn't a concern. The risk comes from concentrated turmeric supplements, which have been linked to cases of liver toxicity severe enough to cause jaundice, or yellowing of the skin.
Supplement manufacturers often pack high doses of turmeric into a single capsule and add black pepper extract to boost absorption beyond what the body would ever get from food. “This is the trouble of anything that we try to manipulate—there are consequences when we do this, and I think we don't recognize it,” the orthopedic surgeon said. Turmeric itself isn't the problem; the concentrated, high-dose supplement form is.
4. St. John's Wort
Often taken to support mood and ease anxiety, St. John's Wort carries what physicians describe as one of the highest risks of interfering with other medications of any supplement on the market. It can make prescription drugs less effective or cause their levels to build up too high in the body.
“If it's strong enough to affect your mood, then it's probably strong enough to interact with other medications you're taking,” the functional medicine physician noted. Patients are urged to disclose every supplement they take to their doctor or pharmacist, since unexplained symptoms or medications that suddenly stop working can sometimes be traced back to an undisclosed herbal supplement like this one.
5. Zinc
Zinc's popularity surged during the pandemic for its role in immune function, and it's also essential for DNA synthesis, nerve function, and wound healing. But taking too much zinc can cause the body to displace copper during absorption, leading to a copper deficiency.
Even though the body needs only trace amounts of copper, that small amount is essential for producing red blood cells—meaning excess zinc can, somewhat ironically, contribute to anemia. It can also affect nerve function. Like several supplements on this list, zinc is easy to unknowingly “stack” across multiple products, from multivitamins to cold remedies.
6. Iron
Iron is essential for hemoglobin, the molecule that carries oxygen through the bloodstream, and those who are genuinely iron deficient should take it as directed. But excess iron accumulates in the body and can be deposited in organs, in addition to causing constipation, nausea, and vomiting.
Physicians caution against starting iron supplements without medical guidance. “If you're iron deficient, you need to find out what the cause is,” the orthopedic surgeon said, noting that unexplained low iron can sometimes signal an ulcer, gastritis, or even a bleeding cancer. Low hemoglobin, he added, is one of the first things doctors want to rule out as a possible sign of cancer.
7. Biotin
Marketed heavily for hair, skin, and nail health, biotin supplements often come in doses far higher than the body needs. Beyond the aesthetic marketing, there's a more serious concern: high biotin levels can interfere with a common blood test called troponin, which is used to diagnose heart attacks.
That interference can produce a falsely negative result—potentially sending someone home from the emergency room despite having a cardiac event. “Doc, I'm having a heart attack. Nope, your blood work's good. Go home,” the functional medicine physician joked grimly. “Sorry about your biotin—your skin and nails look good, though.”
8. Iodine and Kelp
Kelp, seaweed, and iodine supplements are often taken to support thyroid function, since iodine is essential for producing the hormones T3 and T4, which regulate metabolism. But more isn't better here either. Taking too much iodine can push the thyroid toward either an underactive or an overactive state, depending on individual susceptibility.
“It just messes with the hormone production process,” the orthopedic surgeon said, adding that iodine is another supplement that can be unintentionally stacked through multiple sources, including sea-vegetable-based multivitamins.
9. Vitamin A
Vitamin A is another fat-soluble vitamin, alongside D, E, and K, meaning it accumulates in the body rather than being flushed out. Like B6, high doses of vitamin A can be neurotoxic. It's also unsafe in pregnancy, where high doses have been linked to birth defects, as well as to blurred vision at toxic levels.
Physicians note that this applies specifically to pre-formed vitamin A, such as retinol, rather than beta-carotene from food sources. Chronically high doses can also affect liver function, adding it to the growing list of supplements that can quietly stress the liver.
10. Red Yeast Rice
Often marketed as a natural alternative to statin medications for high cholesterol, red yeast rice contains a compound called monacolin K—which is chemically identical to the prescription statin lovastatin. In other words, many people taking red yeast rice to avoid a statin are, unknowingly, taking a statin.
The critical difference is quality control. Pharmaceutical statins are manufactured within a tightly regulated dosage tolerance, typically within a few percent of the labeled amount. Supplements are not held to that standard, so the actual dose of monacolin K in red yeast rice can vary dramatically from bottle to bottle—and even batch to batch. “It's a little bit rolling the dice,” the functional medicine physician said. Because it functions as a statin, red yeast rice can carry the same side effects, including blood sugar changes, muscle cramping, and liver toxicity, and should be monitored the same way a prescription statin would be.
Honorable Mentions
A handful of other supplements didn't make the top 10 but are worth a mention. Niacin, often used to try to lower cholesterol, is well known for causing an uncomfortable skin flushing reaction in high doses. Vitamin E, another fat-soluble vitamin, can increase the risk of bleeding when taken in excess. Green tea extract—far more concentrated than a cup of brewed green tea or matcha—has occasionally been linked to liver toxicity. And broad, vaguely labeled “herbal supplements,” especially those marketed for sex drive or fat burning, are a category to approach with particular caution, since they may contain undisclosed ingredients not listed on the label.
The Bottom Line
None of this means supplements are inherently dangerous, physicians stress. The concern is less about the supplements themselves and more about the assumption that if a little is beneficial, a lot must be better. Anyone taking a supplement because of a confirmed deficiency, a restricted diet, or a physician's recommendation should generally continue doing so.
The bigger risk lies in taking high doses without medical guidance, or unknowingly “stacking” the same ingredient across multiple products—a multivitamin, a fortified energy drink, and a post-workout supplement, for example, can all quietly contain the same vitamin or mineral. Disclosing every supplement to a doctor or pharmacist, rather than assuming they're automatically harmless because they're “natural,” remains one of the simplest ways to avoid an unwanted surprise.
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