The amount of caffeine in diet supplements varies widely, and product labels are often inaccurate or have no caffeine information at all, according to a US study.

The caffeine doses probably wouldn't be a problem on their own, but they may cause issues when the pills or powders are combined with energy drinks, coffee and other high-caffeine food and beverages, said researchers, whose report appeared in Jama Internal Medicine.

"Consumers really have no idea when they're purchasing supplements what's in them, even if they carefully read the label," said Pieter Cohen from Harvard Medical School in Boston, who worked on the study.

He and his colleagues analysed the caffeine content of 31 dietary supplements that are known to have added caffeine or herbal ingredients that naturally contain caffeine, and are sold on military bases.

Eleven of the supplements listed herbal ingredients, and all of those had no caffeine or only minimal traces, the research team reported.

Among the other 20 products, nine had labels with accurate caffeine information. Another five had varying caffeine contents that were either much lower or higher than the amount listed on the label.

The remaining six products did not have caffeine levels on their labels, but had very high amounts according to the chemical analysis - between 210 and 310 milligrams per serving. In comparison, an eight-ounce cup of coffee has about 100 mg of caffeine.

Those levels are especially worrisome for military service members abroad, Cohen said, because side effects of caffeine such as tremors and anxiety may hit them extra hard due to the stressful environment.

Too much caffeine, according to Cohen, "could push one over from just being a little on edge to having a full-blown panic attack."

One limitation of the study was that the researchers only tested one of each supplement. Because of that, it wouldn't be right to call out any single company on its products, said Cohen, who is also an internist at Cambridge Health Alliance.

What stood out to him was how common inaccurate labeling or lack of information was, across the board.

A spokesperson for GNC, which produces and sells die and nutritional supplements, told Reuters Health in an email, "We believe that all GNC brand labeling is appropriate as to ingredients and dosage.

"GNC does, like many other retailers, sell third party products and questions on their labeling practices should be addressed directly to those manufacturers." – Reuters