5 supplements you shouldn't take to prevent cancer, according to an oncology dietitian

It’s tempting to believe that a handful of pills can shield us from glaring diseases. Especially, in today’s wellness-obsessed world, it’s easy to reach for the latest supplement promising protection from cancer. But all that glitters isn’t gold. Likewise, all the supplements – even though they seem like they might help boost our health – do us more harm than good.

And that is why practicing caution is extremely important.

In fact, professional healthcare providers often encourage people to get their nutrients from food, not supplements.

Hillary Wright, M.Ed., RD, a senior oncology dietitian at Dana-Farber Cancer Institute, told Eating Well, “Overwhelmingly, studies on supplements rarely pan out.”

Moreover, oncology dietitians caution that high‑dose supplements often backfire – some even increase cancer risk or interfere with treatments.

As per Wright, “It can be easier to take a pill than think about how to incorporate more fruits and vegetables into your diet.” However, as oncology dietitians caution, what seems like a harmless dose of vitamins or plant pow­ders can sometimes backfire – interfering with treatment, boosting risk, or offering no real benefit.

Here are five supplements popular for purported cancer prevention that actually carry warnings. Read on to understand why a food-first approach is safer – and why these five supplements are best left off your shelf unless medically indicated.


Beta‑carotene (and high‑dose antioxidant vitamins)

Large clinical trials have shown that supplemental beta‑carotene increased lung cancer risk by about 16% in smokers, and even stomach cancer in some groups. Antioxidants like vitamin E and beta‑carotene may also blunt cancer therapies’ effectiveness by neutralizing oxidative stress, one of their key mechanisms. The US Preventive Services Task Force (USPSTF) advises against using beta‑carotene or vitamin E supplements for cancer prevention.


As per Wright, “The interest in beta carotene came from the observation that people who eat more fruits and vegetables seem to get less lung cancer. However, beta carotene is one singular component of countless compounds that someone’s body is exposed to if they eat more fruits and vegetables.”


Vitamin C (supplement form)

Despite its popularity, vitamin C supplements show no clear benefit for cancer prevention. Some studies raise concerns that high oral doses (≥1 g) may increase kidney stones and, more worryingly, interfere with chemotherapy, such as bortezomib. Cancer.gov recommends obtaining vitamin C through foods, not pills, especially during therapy.

As per Wright, “There’s been an interest in vitamin C ‘boosting’ the immune system for decades. I haven’t read any science that backs that up across the board. There is a lack of evidence suggesting that we should take daily supplemental vitamin C.”

However, eating foods high in vitamin C is a good idea – and in fact, a C-rich diet has been associated with a lower incidence of several cancers, including breast, cervical, endometrial, esophageal, lung, pancreatic, and prostate cancer.



Vitamin E

The landmark SELECT trial – including over 35,000 men – was halted early after finding no benefit in prostate cancer prevention, followed by later data showing a 17% increased risk of prostate cancer among vitamin E users. Although there have been studies done on the effects of vitamin E supplements on some cancers, the results have been mixed. Disappointingly, there is not enough evidence that taking vitamin E will help you prevent cancer, and it can increase your risk of certain cancers, particularly prostate cancer, when consumed in large doses. The USPSTF now recommends against supplementing with vitamin E for cancer or cardiovascular disease prevention. High doses can also interfere with chemotherapy agents like tamoxifen.


Selenium

Though early studies hinted at benefits in selenium-deficient populations, large, rigorous trials – including SELECT – found no overall protective effect, and some data suggest potential harm in men with adequate baseline selenium. In the SELECT trial, selenium was even linked to increased high-grade prostate tumors in certain groups. Rather than blanket supplementation, selenium should be considered only when deficiency is confirmed, ideally by measuring serum levels.



Folic acid / High‑dose folate

While dietary folate is essential, supplemental folic acid at high doses (1,000–2,500 µg/day) has been linked to a 24% rise in prostate cancer risk, along with concerns about fueling existing precancerous lesions. The artificial form in supplements can exceed what you’d get from food alone, raising potential issues. Unless prescribed (e.g., during pregnancy or deficiency), fortified folic acid supplements should be avoided.


Why the food-first approach is healthier

Food sources are safe, but supplements carry real risks, especially during cancer treatment. Foods contain fibers, antioxidants, and bioactive compounds that work together in ways supplements can’t replicate. On the other hand, Supplements often deliver unnaturally high levels that may disrupt the body's finely tuned systems and can interfere with chemotherapy, hormone agents, and radiation therapies – and they include herbal supplements as well. Turmeric – though it is beneficial when used in the kitchen – high-dose curcumin supplements may interfere with hormone therapies such as tamoxifen.

Furthermore, vitamin D for deficiency, folate during pregnancy – all these supplements should be used following blood tests and medical advice.

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