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COSMOS, like many trials, was able to test two interventions for essentially the price of one. The first: cocoa flavanol supplements. The second: multivitamins.

“We were interested in trying to replicate the findings from the Physicians’ Health Study II, which we had completed about a decade earlier,” says JoAnn Manson, chief of preventive medicine at Brigham and Women’s Hospital and professor of medicine at Harvard Medical School. “It found a small, 8 percent reduction in total cancers in men aged 50 or older who were randomly assigned to take a multivitamin."

(Put another way, roughly 80 men would have to take the multi for 11 years to prevent one cancer.)

So COSMOS randomly assigned 21,442 older people to take either a placebo or a Centrum Silver multivitamin. (Centrum’s manufacturer supplied the pills.)

The COSMOS results for cancer

After 3½ years, the multivitamin takers had no lower risk of cancer than the placebo takers. However, it’s possible that COSMOS was too short to see an impact on cancer, which can take many years to be detected.

“The Physicians’ Health Study II was an 11-year trial, and it wasn’t possible to extend COSMOS for a decade,” Manson points out.