Derek Beres, BigThink:

The National Institute of Health spent $2.4 billion studying vitamins and supplements only to find out they really don’t work. As Pieter A. Cohen writes in JAMA:

During the past 2 decades, a steady stream of high-quality studies evaluating dietary supplements has yielded predominantly disappointing results about potential health benefits, whereas evidence of harm has continued to accumulate.

This includes clinical trials showing that vitamin E, once promoted as heart healthy, actually increases your risk of heart failure and prostate cancer. Multivitamins do not prevent cancer and heart disease; St John’s wort will do nothing for your depression; Echinacea is no match for the common cold. In smokers, beta-carotene increases the risk of lung cancer.

And

In study after study Offit shows that cancer and heart disease rates increase with the consumption of vitamins and supplements. A few examples:

  • A 1996 study in Seattle of 18,000 people showed that people exposed to asbestos who were taking megavitamins with large doses of vitamin A and beta-carotene were 28 percent more at risk of developing lung cancer and 17 percent more at risk for developing heart disease.
  • A 2004 study in Copenhagen conducted 14 randomized trials with 170,000 people and discovered that those taking large amounts of vitamins A, C, E, and beta-carotene were more likely to develop intestinal cancer.
  • A 2005 study at John Hopkins School of Medicine performed a meta-analysis of 19 studies with over 136,000 people. Those taking megavitamins were at an increased risk of early death.
  • Another 2005 study of 9,000 people published in JAMA found increased risks of cancer and heart disease in those taking large doses of vitamin E.
  • A 2011 study at the Cleveland Clinic involving 36,000 men found a 17 percent increased risk of prostate cancer in those consuming vitamin E and/or selenium.