More than half the Canadian population regularly uses vitamins and minerals in order to stay healthy, according to Health Canada. Yet most are likely not aware that high doses of many common vitamins can increase the risk of cancer or death.

“Vitamins have become synonymous with health, but there’s this false idea that, if a little bit is good, a lot has to be better,” says Dr. Tim Byers, associate dean at the Colorado School of Public Health. Byers challenged this idea recently at the American Association for Cancer Research’s annual meeting. “We now have direct evidence that a lot is not better. In fact, a lot is worse and can create health problems,” he said.

There is no doubt the body needs a minimum amount of vitamins to function normally. Deficiencies in vitamin C can lead to scurvy, and inadequate vitamin D can cause the bony deformities of rickets in children, though these diseases are rarely seen in Western countries nowadays, thanks largely to widespread improvements in nutrition.

There are some valid clinical uses for vitamin therapy, such as folic acid given in pregnancy to prevent neural tube defects, or vitamin K given prophylactically to prevent hemorrhagic disease in newborns. Vitamin B12 is the treatment for patients with pernicious anemia, and iron supplements are often given to patients with iron deficiency.

But most people who take vitamins don’t have any of these medical problems. In fact, most of them are completely healthy and take vitamins not to treat sickness, but to prevent it. So the first question Byers asked himself when he set out to review 30 years’ worth of research conducted on vitamins and cancer was: Can vitamins actually prevent disease? He presented his findings at the American Association for Cancer Research meeting.

The 1996 Physicians’ Health Study, he pointed out, found there’s no evidence to show that beta-carotene (the dietary form of vitamin A) prevents cancer. In 2009, the Physicians’ Health Study II found that vitamin C and vitamin E had no protective effect, either. The examination of a number of trials, specifically, in women, again showed no benefit for tens of thousands of patients over 10 years. Study after study showed that vitamins C, D and E did not lower the risk of cancer.

But hidden in the mass of data that Byers presented at the conference are worrying signs that vitamins might actually increase people’s cancer risk. Two other large randomized trials found that high doses of beta-carotene increased the risk of lung cancer in both male and female smokers. Another trial that studied 35,000 men in Canada and the U.S. found a link between high doses of vitamin E and prostate cancer in men.

Moreover, the findings from these various studies were reviewed by the Cochrane Collaboration, an international group of researchers, who, after analyzing the data, found no evidence to support the use of vitamin therapy, and corroborated that there seemed to be an increased risk of dying with beta-carotene and vitamin E supplements. That position was later echoed by the U.S. Preventive Services Task Force.

“After so many trials and so many studies, I think we can be very confident that these compounds are not having much benefit and are likely doing harm,” says Dr. Eliseo Guallar of Johns Hopkins University. But Guallar believes most of the risk is due to the use of supraphysiological doses, or doses above and beyond what the body actually requires.

Water-soluble vitamins, such as vitamin C and the B-complex vitamins, will simply be filtered by the kidneys and passed in the urine, once the blood levels rise above a certain threshold. Fat-soluble vitamins, such as vitamins A, D, E and K, may pose more of a problem, accumulating in the soft tissues of the body and potentially causing toxicity at higher doses.

“Most [vitamin E] supplements are 400 IU, which is 20 times what you get with diet,” Guallar explains. That is also 20 times more than you may really need. Research he conducted with other colleagues at Johns Hopkins showed that the risk with vitamin E followed a dose-response pattern, where the danger only became visible at these higher doses. In other words, people taking higher doses have a greater risk of dying.

Byers agrees that the concern with supplements is the dose. “My problem is the people taking handfuls [of vitamin supplements], or the manufacturers that are putting incredibly high doses into single pills in a completely unregulated way,” he says.

The results of these trials have been seen for diseases besides cancer, as well. In 2013, a series of articles in the Annals of Internal Medicine showed that daily multivitamins did not lower people’s risk of developing heart disease or Alzheimer’s. It led Guallar and a number of other colleagues to write an editorial in the same issue titled, “Enough is enough: Stop wasting money on vitamin and mineral supplements.”

For well-nourished adults, Guallar says, vitamins are, at best, ineffective. “I wish we had a magic pill that could decrease disease,” he says, “but these vitamins are not that pill.”