The Center for Science in the Public Interest (CSPI) has notified Bayer Healthcare that it will sue the company if it continues to claim that the selenium in its One A Day vitamins may reduce men’s risk of prostate cancer, the health group announced today.

Advertisements and labels for One A Day Men’s 50+ Advantage and One A Day Men’s Health Formula multivitamins claim that “emerging research” suggests that selenium may reduce the risk of prostate cancer. “Did you know that there are more new cases of prostate cancer each year than any other cancer?” intones the narrator one such radio ad. “Now there is something you can do.”

But CSPIstands behind leading prostate cancer researchers who say there is scant evidence to support such a claim and have joined the organization in urging FTC to put an immediate stop to the deceptive claims.

“Bayer is exploiting men’s fear of prostate cancer just to sell more pills,” said CSPI senior nutritionist David Schardt. “The largest prostate cancer prevention trial has found that selenium is no more effective than a placebo. Bayer is ripping people off when it suggests otherwise in these dishonest ads.”

A seven-year, $118-million study funded by the National Institutes of Health found last year that selenium does not prevent prostate cancer in healthy men. The Selenium and Vitamin E Cancer Prevention Trial (SELECT) involving 35,000 U.S. and Canadian men was halted in October when researchers determined that selenium was not protecting the men from prostate cancer and may have been causing diabetes in some of them.

The only study to find that selenium might prevent prostate cancer in men was the Nutritional Prevention of Cancer (NPC) trial, which unexpectedly found in 1996 that selenium supplementation seemed to prevent prostate cancer in men with a history of skin cancer. However, two later analyses of the NPC results determined that only a small minority of men may have benefited from selenium supplementation and that selenium almost tripled the risk of developing diabetes. That led to a dramatic warning from the American College of Physicians that “long-term selenium supplementation should not be viewed as harmless and a possibly healthy way to prevent illness.”

In an editorial accompanying publication of the SELECT study results in the Journal of the American Medical Association, Peter Gann of the University of Illinois at Chicago urged that “physicians should not recommend selenium or vitamin E—or any other antioxidant supplements—to their patients for preventing prostate cancer.”

Yet, Bayer still touts selenium’s promise in preventing unspecified prostate “issues” and in reducing prostate cancer risk.

“With these indefensible claims, Bayer is thumbing its nose at FDA, the FTC, and any number of state consumer protection laws,” said CSPI litigation director Steve Gardner. “A courtroom would be treacherous territory for Bayer, whose executives would be committing perjury just by reciting their ads under oath.”

In recent years CSPI’s litigation department has negotiated settlements or voluntary changes to marketing practices with Airborne, Anheuser-Busch, Frito-Lay, Kellogg, Pinnacle Foods, Quaker Oats, and others.

Besides announcing its intention to sue Bayer, CSPI also filed a complaint today with the FTC. That complaint states that because Bayer’s ads have for so long reinforced the false notion that selenium prevents prostate cancer—and because selenium may actually increase the risk of diabetes—the company should be required to run a corrective advertising campaign. (Bayer is now running corrective advertising at the behest of the FDA and state Attorneys General about yet another one of its products, its birth control pill Yaz.)

CSPI says the prostate cancer claims for One A Day supplements for men violate a consent decree the company signed with the FTC in 2007. That year Bayer paid a $3.2 million fine related to weight-loss claims made on behalf of One A Day multivitamin WeightSmart, and agreed not to make unsubstantiated claims in the future.

Separately, some of the most prominent prostate cancer researchers in the United States wrote to the FTC in support of CSPI’s complaint about Bayer’s advertising. The SELECT trial “was the largest individually randomized cancer prevention trial ever conducted, and, given its high rates of adherence and its statistical power, it is unlikely to have missed detecting a benefit of even a very modest size,” wrote the researchers. “Bayer Healthcare is doing a disservice to men by misleading them about a protective role for selenium in prostate cancer.”

Signatories include Peter Gann and Maarten Bosland of the University of Illinois at Chicago, Ed Giovannucci of the Harvard Medical School, Alan Kristal of the Fred Hutchinson Cancer Research Center, William Nelson of the Johns Hopkins Medical School, Tim Byers of the University of Colorado, Larry Kushi of Kaiser-Permanente in Oakland, Lawrence Kolonel of the University of Hawaii, and Michael Thun of the American Cancer Society.