The study, released on Thursday, said Sambazon Organic Amazon Energy drink had 81 milligrams of caffeine per serving.

The privately-held company said the findings were wrong, adding that its drink actually contains 53 milligrams of caffeine per eight-ounce serving.

Consumer Reports said it stands by its findings.

According to the magazine's study released on Thursday, 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 actually contained more caffeine per serving than was listed and by an average amount of over 20 percent.

Energy drinks have come under renewed scrutiny this week after market leader Monster Beverage Corp was sued by the family of a Maryland teenager with a heart condition who died after drinking two of its energy drinks in 24 hours.

(Reporting by Martinne Geller in New York, editing by Gary Crosse)

• A Permanent Economic Emergency