Of course, older people are not the only ones looking for a mental jolt. Teenagers and young adults have popularized heavily-caffeinated drinks with names like Red Bull, Monster, Amp and Full Throttle. Those who love them know they're getting a lift, but few have any idea how much caffeine they are actually drinking. NPR's Allison Aubrey reports.

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ALLISON AUBREY: Skateboarding requires lots of energy. So when 19-year-old Evan Cathcart wants a quick boost, he likes to pick up a can of Red Bull.

Mr. EVAN CATHCART (Skateboarder): Well, I mean, cause I like energy drinks. So, like, just like right now we're at a skate park. (unintelligible) like, fine, let's get some more energy I can skateboard longer. And it tastes good.

AUBREY: Cathcart's skateboarding at a park in Kensington, Maryland, just a stone's throw from a 7-Eleven convenience store where a 12-ounce Red Bull costs about $2. Cathcart says he doesn't drink them everyday, but when he needs a lift, he figures energy drinks are more potent than Mountain Dew or Coke.

Do you have any idea how much caffeine is in that Red Bull?

Mr. CATHCART: Well, it must be quite a bit. Probably more than a soda. I don't have an idea of like exactly how much. I don't really know how it's measured or anything.

AUBREY: Cathcart is hardly alone. Very few consumers have any idea how much they're taking in because the caffeine content is not listed anywhere on the cans of energy drinks. And only a few brands contain caffeine warnings. Toxicology Professor Bruce Goldberger at the University of Florida was curious to know how much was in each drink. So he tested 10 brands and found a huge range of caffeination.

Professor BRUCE GOLDBERGER (Toxicology, University of Florida): One of the energy drinks that's called Hair of the Dog contains no caffeine. It's caffeine-free, totally. And we tested a SoBe No Fear can of energy drink - it's a 16-ounce serving - and it contained a total of 141 milligrams.

AUBREY: Which is basically the equivalent of almost five cans of Coke or three Mountain Dews or a cup and a half of coffee. Goldberger also tested an eight-ounce sugar-free can of Red Bull, which had twice the caffeine of a can of Coke or Pepsi.

Caffeine researchers who've studied the effects of the stimulant on the body say there's nothing alarming about the amount of caffeine in a single can of Red Bull. But Roland Griffiths of Johns Hopkins University says the problem with teens and caffeine is that many don't realize how easy it is to become dependent on the stimulant.

Professor ROLAND GRIFFITHS (Johns Hopkins University): They use it for three and four days in a row and start getting physically dependent, and then all of the sudden have to abruptly quit, they're going to be thrown into withdrawal.

AUBREY: Which can mean headaches, grumpy moods and trouble concentrating. Those who stick with the habit and drink more than their bodies can handle may get a bad case of the jitters. Some people are more sensitive than others. But just like coffee, caffeinated energy drinks are habit forming.

Prof. GRIFFITHS: I think it's important that people recognize that caffeine really is a drug, and they accord it respect as a drug.

AUBREY: But teens and young adults seem to be drinking more of the stuff than ever. Sales of energy drinks reached almost $2 billion last year. As the market expands, some brands are pushing caffeine levels to new highs. One drink made by Redux Beverages called Cocaine has three times the caffeine of Red Bull. To get their name out, the company's Hannah Kirby says they sponsor contests where teens and young adults submit wacky videos they've put up on Web sites like YouTube or MySpace.

Ms. HANNAH KIRBY (Redux Beverages): We send them out a T-shirt, we send them out a case of the drink. And, you know, we've kind of done a lot of stealth marketing.

AUBREY: So even if you'd never heard about these contests or seen an add for one of these drinks, your teens probably have. So now might be the time for a caffeine chat about how much is too much.

Allison Aubrey, NPR News, Washington.

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INSKEEP: You could put Red Bull up against Mountain Dew, up against Starbucks Coffee and get a comparison of caffeine levels by going to our Web site,

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