You drink them cold, but energy drinks are hot. U.S. consumers spent $744 million on those caffeinated beverages during the year ending June 17, 2007, a 34 percent increase over the previous year, says Information Resources, a market-research firm. With names like Rockstar, Full Throttle, and No Fear, they may appeal especially to young adults.

"Energy drink" sounds healthful, but most contain multiple stimulants, making them a poor choice for young children and pregnant women; and they can be risky if mixed with alcohol. We think the government should require companies to label caffeine content, but it doesn't, so you often have no idea how much you're getting.

Our experts tested 12 carbonated energy drinks (9 regular and 3 low-calorie) for caffeine content and taste.

What we found. Caffeine levels per 8-ounce serving ranged from 50 milligrams to 145 mg. By comparison, a cup of coffee has roughly 100 mg; 8 ounces of Coke, about 35 mg. Caffeine intake up to about 300 mg a day is considered OK for most adults; children should be limited to well under 100 mg. But most tested energy drinks have more than 8 ounces per can or bottle. Anyone who guzzles the whole container consumes up to 200 mg of caffeine and up to 260 calories.

All the drinks tasted sweet, and many were a bit bitter. Sugar-free versions of Full Throttle, Red Bull, Rockstar, and Sobe had an artificial-sweetener flavor.

Most of the drinks list at least one stimulant ingredient. Green tea and guarana supply caffeine, as does yerba mate, found in Steaz. Ginseng might amplify caffeine's effects. Other ingredients include taurine, which some studies suggest improves mental performance, and glucoronolactone, which might yield a temporary "sugar buzz." Celsius and Enviga claim to burn calories, and in a few manufacturer-funded studies, healthy adults worked off about 100 calories more on average after drinking them than after drinking a placebo. (Connecticut's attorney general has asked Enviga for proof.) Other claims can be nonsensical. Red Bull says it "improves performance, especially during times of increased stress or strain," but high caffeine levels can actually cause anxiety.

CR's take. An occasional energy drink is fine for most people, but don't over-indulge. Check the Ratings (available to subscribers) for caffeine level, taste, and cost (approximate retail).