More than 500 new energy drinks launched worldwide this year, and coffee fans are probably too old to understand why.

Energy drinks aren't merely popular with young people. They attract fan mail on their own MySpace pages. They spawn urban legends. They get reviewed by bloggers. And they taste like carbonated cough syrup.

|QUOTE|Vying for the dollars of teenagers with promises of weight loss, increased endurance and legal highs, the new products join top-sellers Red Bull, Monster and Rockstar to make up a $3.4 billion-a-year industry that grew by 80 percent last year.

Nutritionists warn that the drinks, laden with caffeine and sugar, can hook kids on an unhealthy jolt-and-crash cycle. The caffeine comes from multiple sources, making it hard to tell how much the drinks contain. Some have B vitamins, which when taken in megadoses can cause rapid heartbeat, and numbness and tingling in the hands and feet.

But the biggest worry is how some teens use the drinks. Some report downing several cans in a row to get a buzz, and a new study found a surprising number of poison-center calls from young people getting sick from too much caffeine.

"The truth is, we don't know what kind of effects these ingredients can have," Dr. Sandra Braganza, nutrition expert at the Children's Hospital at Montefiore in New York, said of taurine, glucuronolactone and guarana. "We have to start doing more studies on this."

A surprising number of caffeine overdose cases have also been reported earlier this month in a Chicago poison control center. Twelve percent of the 265 cases found required a trip to the hospital; the average age was 21.

"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 Dr. Danielle McCarthy of Northwestern University, who was responsible for the study at the poison control centre.

"Caffeine is a drug and should be treated with caution, as any drug is." she added.