One of the greatest triumphs of marketing over evidence was the incredible rise of vitamin supplement use in the 20th century. Supplement makers successfully created a “health halo” around vitamins, and taking your vitamins became a virtue, something mothers told their children to do. The evidence, however, does not tell such a simple story.

In recent years it has become increasingly apparent that there are unintended consequences to taking vitamin supplements, and in fact there may be a net negative health effect. This is especially true for those who are healthy and don’t need vitamins, and for those who exceed the recommend dosages.

A recent review of the last 20 years of literature on the subject, presented at the American Association for Cancer Research 2015 meeting, found an overall increased risk of cancer among vitamin users. Dr. Tim Byers presented the study, which echoes the result of a 2012 review that he and others published. He specifically refers to two famous studies showing an increased risk of cancer from vitamins.

The 2011 SELECT trial found an overall increased risk of prostate cancer among men taking vitamin E.

It has also been shown, first in a 1994 NEJM study, that high doses of beta carotene increase the risk of lung cancer in smokers and those exposed to asbestos.

In fact, antioxidants in general lack evidence for an overall health benefit and in high doses may increase cancer and overall mortality.

Folic acid and cancer risk is a more complex topic. There is evidence that taking folate reduces the risk of breast and possibly colon cancer. There is a “U” shaped relationship between dose and cancer risk, however:

Women with daily dietary folate intake between 153 and 400 μg showed a significant reduced breast cancer risk compared with those <153 μg, but not for those >400 μg.

There is preliminary evidence that higher doses of folate may increase cancer risk.

There are several possible mechanisms by which excess vitamin intake may promote cancer. Cancer cells are metabolically very active. Taking more vitamins than is necessary for healthy cells may only serve to feed cancerous cells, promoting their growth. Elevated levels of vitamins, exceeding what the body need for normal metabolism, may also result in the creation of alternate downstream metabolites that have negative health consequences. Further, taking megadoses of a vitamin may alter the body’s natural homeostasis with negative consequence. This is likely true with antioxidants – oxidative compounds are used by the immune system and act as signals for the creation of protective proteins. There is a balance between oxidants and anti-oxidants, and shifting that balance is not beneficial.

There are also possible unintended consequences. The evidence is fairly clear that having a healthful and well-rounded diet has health benefits. Taking vitamins does not replace having a good diet. However, taking vitamins may lead to a false sense of security, justifying a less healthful diet with net negative health consequences.


Dr. Byers gave a reasonable summation of the evidence:

This is not to say that people need to be afraid of taking vitamins and minerals. If taken at the correct dosage multivitamins can be good for you. But there is no substitute for good food.

My reading of the evidence leads to several specific recommendations:

  • Do not use vitamins as a substitute for an overall healthful diet. Try to get your vitamins and minerals through a well-rounded diet with sufficient fruits and vegetables.
  • Routine supplementation in healthy individuals is unnecessary.
  • Do not take megadoses or even exceed recommended doses of vitamins or minerals. The evidence suggests this can be harmful.
  • Targeted supplementation of specific vitamins at correct doses in specific individuals or populations can be beneficial.
  • For targeted supplementation (such as folic acid in women of child-bearing age) follow accepted guidelines, or the advice of your physician. Often physicians will directly measure the blood level of specific vitamins and supplement accordingly.

Perhaps the greatest harm of the supplement industry over the last half-century has been the cultivation of the idea that vitamin and mineral supplements are harmless, that if some is good then more is better, and that the public can determine their own supplement needs based on industry marketing and magazine articles (and now websites).

It turns out that none of these assumptions are true. There is a risk to excess vitamin use. Determining who should receive what dose of which vitamin is also a bit complex and nuanced, and perhaps a little professional guidance is in order. It’s quite possible that vitamin supplement use, in addition to costing billions of dollar per year, has been a net health negative. We can save money and ensure that it is a net health positive with more rational and evidence-based supplementation.


Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.