The study has some holes, however. For one, it didn't identify the 31 supplements that it examined. The researchers said only that they're the most popular supplements sold as pills on military bases with labels that indicate that they include either caffeine or herbal ingredients that include caffeine.
Of the 31 supplements, 20 listed caffeine on their labels. Of those 20, only nine correctly listed the amount, according to the researchers. Five listed amounts between 27 percent and 113 percent off from the actual amount.
Six products listed caffeine as an ingredient but didn't say how much. The researchers found that they had 210 to 310 milligrams per serving -- the same amount that is in two to three cups of coffee.
People often drink coffee or take energy supplements to become more alert, and Cohen said it's true that the caffeine in two to three cups of coffee can improve performance. But people lose the boost at about five cups, he said.
What to do? Higgins, the Texas cardiologist, said manufacturers need to be required to state properly how much caffeine is in supplements, and the amounts need to be independently verified.
Another expert said that giving consumers consistent, accurate information could benefit their health.
"If consumers had a better idea about how much caffeine they were getting from various sources -- from energy drinks and supplements -- they would count it up. They would take notice and realize that they may be overdoing it," said pharmacist Philip Gregory, editor of the Natural Medicines Comprehensive Database.
The study appeared in the Jan. 7 issue of the journal JAMA Internal Medicine.
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