The market for energy drinks is worth an annual $3.4 billion, 500 new products were introduced last year, and 31 percent of U.S. teens say they drink the highly caffeinated beverages, the Washington Post reported Oct. 29.

Nutritionists worry that drinks like Red Bull, Monster and Rockstar, which are full of caffeine and sugar and often contain megadoses of B vitamins, are dangerous; overconsumption has sometimes led to calls to poison-control centers for heart palpitations and other problems.

Worldwide sales of market leader Red Bull, launched in 1987, now total 2.5 billion cans in 130 nations. Fierce competition has led to products with ever more provocative names and claims; brands now include Cocaine Energy Drink, Pimpjuice, and Bawls. "We knew we would get noticed against a thousand other energy drinks," said Hannah Kirby of Cocaine, which she and her husband, James, developed. "We knew kids would find it cool, but we also wanted to stress the idea that it's an energy drink, you don't need drugs."

Swedish researchers who studied energy drinks said that they should not be mixed with alcohol, but drinks using Red Bull as a mixer are common, and both Anheuser-Busch and Miller Brewing now have "energy beers" that mix caffeine-based drinks with alcohol.

Experts worry about how little research has been done on typical energy-drink ingredients like taurine, glucuronolactone and guarana. "The truth is, we don't know what kind of effects these ingredients can have," said Sandra Braganza, a pediatrician and nutrition expert at the Children's Hospital at Montefiore in New York. "We have to start doing more studies on this."

Some energy drinks contain up to four times more caffeine than a can of Coke. The drinks are unregulated in the U.S. As competition picks up, some companies are raising serving sizes and caffeine content even higher.

Some experts said the young people should consume no more than one of the drinks per day.

"Young people are taking caffeine to stay awake, or perhaps to get high, and many of them are ending up in the emergency department," said researcher Danielle McCarthy of Northwestern University. "Caffeine is a drug and should be treated with caution, as any drug is."