When it comes to health supplements, there’s so much hype about their potential benefits that it can be hard to separate fact from fiction. While it’s true that vitamins and minerals are essential to health, it’s not true that taking them in pill, capsule, or powder form — especially in megadoses — is necessary or without risks.

For one thing, dietary health supplements can sometimes interact with each other, as well as with over-the-counter (OTC) and prescription medication. In addition, unlike drugs, the U.S. Food & Drug Administration (FDA) is not authorized to review dietary supplements for safety and effectiveness before they are marketed. It’s up to manufacturers to ensure that their products do not contain contaminants or impurities, are properly labeled, and contain what they claim. In other words, the regulation of dietary supplements is much less strict than it is for prescription or OTC drugs.

Yet, according to the FDA, more than half of Americans take herbal or dietary supplements daily, with a report by Grandview Research noting the dietary supplements market was valued at $151.9 billion worldwide in 2021.

Used properly, some health supplements may improve your health, but others can be ineffective or even harmful. For example, a systematic review analyzing the potential effects of nutritional supplements on cardiovascular health, mainly heart attack and stroke, suggests that few supplements help prevent heart disease — only omega-3 fatty acids and folic acid were effective. The same went for dietary changes, except for a low-salt diet.

Other research involving self-reported dietary habits from a group of Americans linked daily doses of more than 1,000 milligrams (mg) of calcium to a higher risk of death from cancer (though other studies, as the National Cancer Institute notes, suggest the opposite). Furthermore, the data showed that people who took in adequate amounts of magnesium, zinc, and vitamins A and K had a lower risk of death — but only if they got those nutrients from food rather than supplements.

"Buyer beware,” warns JoAnn Manson, MD, the chief of the division of preventive medicine at Brigham and Women’s Hospital in Boston and a professor of epidemiology at the Harvard T. H. Chan School of Public Health in Cambridge, Massachusetts. “Many supplements on the market have not been rigorously tested. Very few supplements have shown to be of benefit." And, she says, many carry unsubstantiated health claims.

Confused? National Institutes of Health (NIH) fact sheets can provide detailed information on the benefits and risks of individual vitamins and minerals, as well as herbal supplements. And if you’re managing an underlying health condition (especially if you’re taking medication) or are pregnant or breastfeeding, play it safe and have a conversation with your healthcare team before adding any new supplement to your regimen.

While supplement trends come and go, here are seven supplements that historically have been popular — and in all cases, experts recommend taking them carefully, if at all.

1. Vitamin D: Too Much Can Harm Your Kidneys

Vitamin D promotes calcium absorption in the body, and having enough is central to health and well-being, offering the promise of protecting bones and preventing bone diseases like osteoporosis, per the NIH. Supplemental vitamin D is popular because it’s difficult (if not impossible for some) to get enough from food.

Also, as the Centers for Disease Control and Prevention notes, our bodies make vitamin D when bare skin is exposed to direct sunlight, but increased time spent indoors and widespread use of sunblock, as a necessary way to prevent skin aging and skin cancer, has minimized the amount of vitamin D many of us get from sun exposure.

But vitamin D supplements are a tricky topic. Sometimes, it can seem that guidelines and research contradict one another. The truth is, enthusiasm for vitamin D supplements is outpacing the evidence.

For example, when healthy pre- and postmenopausal women take vitamin D (up to 400 international units, or IU), it does not necessarily prevent them from breaking bones, according to a 2018 U.S. Preventive Services Task Force.

And taking high doses is not a good option. In healthy people, vitamin D blood levels higher than 100 nanograms per milliliter can trigger extra calcium absorption — and lead to muscle pain, mood disorders, abdominal pain, and kidney stones, notes the Cleveland Clinic. It may also raise the risk of heart attack and stroke.

“More is not necessarily better when it comes to micronutrient supplements,” says Dr. Manson.

That said, vitamin D supplements may benefit certain people, including those at risk for deficiency such as individuals who have darker skin, are living with certain health conditions, and older adults, per MedlinePlus. The most recent American Geriatrics Society consensus statement specifically suggests that people older than 65 can help reduce the risk of fractures and falls by supplementing their diet with at least 1,000 IU of vitamin D per day, in addition to taking calcium supplements and eating vitamin D-rich foods.

Anyone can help bolster their vitamin D intake by spending a brief time in the sun without sunblock — about 10 to 15 minutes a day, according to the NIH.

Keep in mind that vitamin D supplements and medications can interact with each other. Meds that don’t mix well with vitamin D include the weight-loss drug orlistat (Xenical, Alli), various statins such as atorvastatin (Lipitor), thiazide diuretics (such as Hygroton, Lozol, and Microzide), and corticosteroids like prednisone (Deltasone, Rayos, Sterapred), according to the NIH.

2. St. John’s Wort: Avoid Drug Interactions

St. John’s wort is a plant used as a tea or in capsules, with purported benefits for depression, attention-deficit hyperactivity disorder, menopause symptoms, insomnia, kidney and lung issues, obsessive-compulsive disorder, wound healing, and more, notes the NIH.

Small studies have shown St. John’s wort to be effective at treating mild depression. For example, a review of short-term studies looked at 27 clinical trials with about 3,800 patients and suggested that the herbal remedy worked as well as certain antidepressants at decreasing symptoms of mild to moderate depression.

But, says Denise Millstine, MD, an internist in the department of integrative medicine at Mayo Clinic in Phoenix, Arizona, “The biggest issue with St. John’s wort is its medication interactions.”

A study found that 28 percent of the time St. John’s wort was prescribed between 1993 and 2010, it was administered in dangerous combinations with antidepressant or anti-anxiety medication, statins, the blood-thinning drug warfarin (Coumadin), or oral contraceptives. For example, combining St. John’s wort with an antidepressant can cause serious complications, including a life-threatening increase in the brain chemical serotonin, according to the National Center for Complementary and Integrative Health.