Are dietary supplements safe? And how well do they work?

Unlike prescription medications and over-the-counter drugs, dietary supplements are not well regulated by the US Food and Drug Administration (FDA). Supplement manufacturers who introduce new ingredients do have to submit a pre-market safety notification that the supplement “will reasonably be expected to be safe” when used as the product label suggests. There is no requirement, though, for efficacy – so unless a dietary supplement is tested in a clinical trial, we really don’t know if it works.

The FDA does not approve the label on a dietary supplement either, but if a claim is made about how the supplement affects the structure or function of the human body (such as “calcium builds stronger bones”), the manufacturer must notify the FDA and include a disclaimer that the FDA has not evaluated that claim.

Supplement manufacturers also need to comply with good manufacturing practices (known as GMP, these are strict requirements that ensure food, drugs and cosmetics are made according to safe, quality standards), and the FDA can inspect manufacturing facilities. Despite this, products with deceptive claims and false labels can end up on shelves. For example, in 2015 the New York State Attorney General’s office found that four out of five herbal supplements sold at four national retailers did not contain any of the herbs mentioned on their labels. The products were often made from little more than cheap fillers like powdered rice, asparagus and houseplants. It ordered them to halt sales of those supplements thereafter.

Can dietary supplements help prevent cancer?

As a general statement, when dietary supplements have been studied rigorously in clinical trials, they have not been shown to prevent cancer.

For example, epidemiological and laboratory studies conducted decades ago initially suggested that taking antioxidants such as beta-carotene could help stave off cancer. Yet, randomised trials enrolling thousands of people, in which approximately half the subjects received dietary supplementation and half received placebo, failed to show that beta-carotene prevents skin cancer recurrence in people with a previous skin cancer, or has any effect on overall cancer rates. Other research has also failed to show that beta-carotene or other antioxidants, such as vitamin C or vitamin E, prevent colorectal adenomas (polyps that can lead to cancer); that those supplements, selenium or vitamin A prevent gastrointestinal cancers; or that beta-carotene or vitamin A prevent lung cancer.

Dietary supplements were not helpful in analyses that separated men from women, either. In one study of over 8000 women, vitamin C and E did not lower overall cancer rates, and in a study of over 36,000 women enrolled in the Women’s Health Initiative, vitamin D or calcium supplementation had no effect on rates of colorectal cancer or on overall cancer rates. Similarly, vitamin E and selenium did not prevent the development of prostate cancer in a study that included over 35,000 healthy men, nor in a study of men with precancerous findings in the prostate.

Can dietary supplements increase cancer risk?

Most dietary supplements are not harmful. Some, however, should be taken with caution, especially in certain populations of people.

In a follow-up analysis of the 35,000 healthy men treated with selenium and/or vitamin E – the study designed to prevent cancer – men taking vitamin E actually had a 17% higher risk of developing prostate cancer than men taking a placebo. Similarly, beta-carotene, which did not do a great job in preventing cancer, was found in one analysis to increase the risk of lung cancer and stomach cancer, particularly among smokers.

Another study, which assigned people to receive folate supplementation or placebo to prevent colon polyps, found that folate was not effective. However, the men who received folate were over 2.5 times as likely to develop prostate cancer compared to those receiving placebo.

Without question, anyone at risk for developing these types of cancers should avoid taking these dietary supplements. But what about people, like my patient, who already have cancer?

In one study of over 1100 women with breast cancer who were receiving chemotherapy, those who took antioxidant dietary supplements before and during treatment had a higher risk of cancer recurrence – though taking a multivitamin had no effect. One possible explanation is that antioxidants may counter some of the cancer-fighting effects of chemotherapy. Dietary supplements can also alter how the body processes chemotherapy, which can theoretically reduce the medication’s efficacy or worsen its side effects.

There’s a lot we still don’t know about the potential benefits or risks of taking dietary supplements, which are not regulated by the FDA. At the very least, tell your doctor about any non-prescription vitamins or supplements you’re taking, so they can make sure they aren’t harming you or interfering with the treatments they do prescribe you.