In recent years, both research on and the availability of vitamin supplements has increased, drawing increased attention to the possibility that a daily pill can enhance health.
In particular, it is a message that is resounding with men. According to the Council for Responsible Nutrition, 53 percent of American adults take a multivitamin supplements, and 50 percent of adult men 18 and older take a multivitamin, according to a 2008 survey.
But preliminary encouraging findings can often hit a wall when they are refuted by larger studies that lead to increased scrutiny on products whose claims overstated the benefits of that early research.
The latest product to enter the crosshairs of a watchdog group is Bayer's One A Day Men's Health Formula supplements, which are promoted as containing "Selenium to support a healthy prostate."
Those claims came under fire from the watchdog group the Center for Science in the Public Interest after two studies on selenium came out in June in the Journal of Clinical Oncology, one of which showed selenium provided no benefit and one which showed that for men with a certain gene variant -- one found in three quarters of men -- selenium supplements made the prostate cancer worse.
The authors of the second study, from the University of California, San Francisco, concluded that their findings "indicate caution against broad use of selenium supplementation for men with prostate cancer."
CSPI released a statement earlier this week in conjunction with a complaint to the Food and Drug Administration.
"Bayer must be stopped from promoting its selenium-containing products as a means of reducing prostate cancer risk and promoting prostate health," senior nutritionist David Schardt said in the statement. "Not only does selenium not prevent cancer, supplementation with selenium may be harmful."
Schardt told ABC News that other supplements have similar problems, such as supplements that contain gingko, whose claims for increasing memory and concentration have not been confirmed by studies.
At this point, he said, the very notion of a men's supplement is faulty.
"Do you need to take a men's supplement?" Schardt said. "A basic adult multivitamin does well for both men and women. The nutrient needs are not that much different."
As of press time, Bayer HealthCare had not responded to a request for comment.
But Andrew Shao, a nutritionist and vice president of scientific affairs for the vitamin trade group the Council for Responsible Nutrition, countered that while claims of selenium preventing prostate cancer have not been proven, they are not completely baseless, having been displayed by some observational studies.
He said that the randomized trials necessary for proof are expensive and, therefore, difficult to conduct. But "That doesn't mean there are no data to support the relationship."
Dr. Donald Hensrud, chair of preventive medicine at the Mayo Clinic, said that without that evidence, there is no clear health benefit from men's supplements.
"One popular men's vitamin contains a little more of certain nutrients to help the prostate, heart, and blood pressure," he said. "However, although there may be some mild evidence associating those nutrients with these conditions, there are no randomized controlled trials showing better health outcomes in subjects who took those nutrients."