Those self-described energy drinks that have flooded the market are loaded with caffeine and should be required to say so on their labels, a group of researchers from the University of Florida contends.

The researchers analyzed the caffeine content of 10 of the best-selling energy drinks along with 19 types of carbonated soda and seven other best-selling commercial beverages.

Many of the energy drinks contained about twice as much caffeine as the typical caffeinated soda beverage, but caffeine content varied widely from brand to brand, and even within brands.

A 12-ounce serving of Coca-Cola Classic, for example contained 29 milligrams of caffeine, compared with 38 milligrams in a Diet Coke and 39.6 milligrams in Diet Coke with Lime.

PepsiCo's Mountain Dew had the most caffeine of any regular soft drink tested, with 45 milligrams per 12-ounce serving.

An 8-ounce cup of regular coffee typically has 100 to 150 milligrams of caffeine.

Energy Drinks and Caffeine

Energy drinks are wildly popular, and it seems that new ones pop up on convenience store and grocery store shelves each week.

But most consumers aren't aware that the vast majority of these drinks rely on large doses of caffeine to boost energy, University of Florida toxicologist Bruce A. Goldberger, PhD, tells WebMD.

The Internet advertising for SoBe's Adrenalin Rush, for example, boasts that the drink is "pure, concentrated energy in an 8.3 fluid ounce can" and it lists the supplements D-Ribose, L-Carnitine, and Taurine as the "natural energizing elements" that help it work.

The drink also has close to 80 milligrams of caffeine, and while this can be found on the web site, it is not highlighted in the ad.

The ad for SoBe's No Fear energy drink reads: "This 16 ounce energy supplement is by far the toughest can on our shelves. After all, if it were a car you'd be scared to drive it."

No Fear had the most caffeine of any of energy drinks tested by University of Florida researchers, with 141 milligrams per 16-ounce serving. The best-selling Red Bull brand had about 67 milligrams of caffeine per 8-ounce can, while the 8-ounce Red Devil brand had about 42 milligrams.

Of the commercially available coffee drinks tested, Starbucks' Doubleshot had the most caffeine, with 105 milligrams per 6.5-ounce serving, while the coffee company's popular Frappuccino Mocha and Frappuccino Vanilla drinks had 72 milligrams and 74 milligrams of caffeine, respectively.

The caffeine content of energy drinks and commercial coffee beverages is not regulated by the FDA, and the amount of caffeine in most of these beverages tested in the study exceeded the maximum allowance for carbonated cola beverages.

Caffeine and Health

Excessive caffeine has been linked to medical complications ranging from interrupted sleep to headaches to women giving birth to smaller babies. Caffeine has also been linked to increasing heart rate and blood pressure, which poses a potential conflict to those with certain medical problems.

Dietitian Cynthia Sass, RD, says caffeine's effect on the body varies from person to person, and that is another reason why clear labeling is needed.

"Some people can have a really strong cup of coffee and go right to sleep and other people get that jittery, nervous, overstimulated feeling from the same amount of caffeine," she says.

Knowing how much caffeine is in a particular product could help people make better decisions about whether or not to consume it, she says.

She adds that just as with other stimulants, using caffeine to combat fatigue may make you feel better temporarily, but you pay for it later.

"When you are fatigued your body needs sleep and you aren't going to function well until you get it," she says. "Using a stimulant like caffeine is a temporary band-aid to the problem."

Labeling Caffeine Content

While the FDA requires commercial beverage manufacturers to list the presence of caffeine on their labels, it doesn't require them to list how much caffeine a product contains.

That should change, Goldberger says.

"We think these beverages should be clearly labeled with the caffeine content listed just as other nutrients are listed," he says.

Johns Hopkins professor of behavioral biology Roland Griffiths, PhD, agrees. Griffiths has been studying the effect of caffeine on the body for many years, and he says the stimulant is the most widely used mood-altering drug in the world.

Griffiths says energy drink consumers are being misled by advertising for the products.

"The ads give people the idea that they are getting a cocktail of various ingredients fine-tuned to synergistically enhance energy," he says. "As far as I can tell, this is bogus. The effects of these drinks are largely due to the presence of added caffeine, and the magnitude of the effect is completely caffeine-dose dependent."