The study conducted by Auburn University in Alabama also found that caffeine content not only varies widely between brands, but marked differences are found within a brand.
Urging the cola manufacturers to include labels on packaging that provide the buyer with the caffeine, the researchers, along with consumer advocates, say consumer has a right to know what he is buying.
Most cans and bottles of soda don't provide with caffeine amounts but many national brand beverage companies including Coca-Cola Co., based in Atlanta, and Purchase, New York-based PepsiCo Inc. are now aiming to do that.
Most national brands however provide lists of the amount of caffeine in their products on their Web sites.
The Food and Drug Administration does not limit the amount of caffeine in foods, but according to a FDA spokeswoman, 0.02 percent caffeine content is considered as safe for cola-type beverages. That makes about 72 milligrams of caffeine "safe" for a 340-millilitre soft drink.
However, the latest study found caffeine content in 340-millilitre sodas ranged from 4.9 milligrams for a store brand of cola to 74 milligrams in Vault Zero, a citrus drink.
After studying the caffeine contents of 56 national brand and 75 store brand carbonated drinks, the lead researchers Leonard Bell and co-author Ken-Hong Chou found that Coca-Cola contained 33.9 milligrams, Diet Pepsi-36.7 milligrams and Pepsi-38.9 milligrams.
The brands with higher contents include Dr. Pepper -42.6 milligrams, Diet Dr. Pepper- 44.1 milligrams and Diet Coke-46.3 milligrams.
The highest caffeine contents were found in Mountain Dew 54.8 milligrams and Diet Mountain Dew that contained 55.2 milligrams of caffeine.
The results were published in the August issue of the Journal of Food Science.
