Often, preliminary studies fuel irrational exuberance about a promising dietary supplement, leading millions of people to buy in to the trend. Many never stop. They continue even though more rigorous studies — which can take many years to complete — almost never find that vitamins prevent disease, and in some cases cause harm.

“The enthusiasm does tend to outpace the evidence,” said Dr. JoAnn Manson, chief of preventive medicine at Boston’s Brigham and Women’s Hospital.

There’s no conclusive evidence that dietary supplements prevent chronic disease in the average American, Dr. Manson said. And while a handful of vitamin and mineral studies have had positive results, those findings haven’t been strong enough to recommend supplements to the general American public, she said.