According to a study released on Thursday by the magazine, 11 of the 27 top-selling energy drinks in the United States do not specify the amount of caffeine in their beverages.

Of the 16 drinks that did list a specific caffeine amount, five had more caffeine per serving than was listed and the average amount over was more than 20 percent.

The study comes fast on the heels of news that U.S. health regulators are investigating reports of five deaths that may be associated with Monster Beverage Corp's Monster Energy drink.

At the same time Monster, maker of the top-selling energy drink in the United States, is being sued by the family of a 14-year-old girl with a heart condition who died after drinking two Monster Energy drinks in a 24-hour period.

The lawsuit and reports of other deaths could escalate calls from critics including two U.S. senators and the New York attorney general about the safety of the beverages and the way they are marketed.

CAFFEINE LEVEL NOT REQUIRED

Aside from companies not wanting to give away their secret recipes, Consumer Reports said there was another reason why some beverage labels do not reveal exact caffeine levels.

"There is no legal or commercial business requirement to do so," a Monster Beverage official told Consumer Reports. "And because our products are completely safe, and the actual numbers are not meaningful to most consumers."

Caffeine levels in the drinks tested ranged from about 6 milligrams per serving for 5-Hour Energy Decaf, made by Living Essentials, to 242 milligrams for 5-Hour Energy Extra Strength, the report found.

The drinks that Consumer Reports found that contained more caffeine than was listed on their labels included Arizona Energy, Clif Shot Turbo Energy Gel and Sambazon Organic Amazon Energy, as well as Dr Pepper Snapple Group Inc's Venom Energy and Nestle Jamba, sold by a partnership of Nestle and Jamba Inc.

One sample of its Archer Farms Energy Drink Juice Infused beverage had about 70 percent less caffeine than advertised, the report found. Archer Farms is the private label of retailer Target Corp.

None of the companies were immediately available to comment.

(Reporting By Martinne Geller in New York; Editing by Maureen Bavdek)

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