"I get one of these to hold me up," Diaz, 27, said one night this week as he paid for his 8.3-ounce can of Red Bull at Dairy Del, a convenience store in Hartsdale, N.Y.

Diaz is typical of the young Type A consumers whose lust to jam more activity and accomplishment into each day has helped turn energy drinks into a lucrative niche for the beverage companies that jumped into the market early.

Sales of energy drinks with edgy names like AMP, 180 and Blue Ox exploded to $1 billion last year and are expected to increase an additional 40 percent to 50 percent this year.

The drinks, which usually contain high concentrations of caffeine and taurine, another stimulant to jolt the nervous system, are a fad with college students cramming for exams, bleary-eyed truck drivers and weekend athletes.

Nightclub patrons order the drinks mixed with vodka to produce just the right combination of lift and buzz - a practice some doctors say might pose a health risk.

Amber Mecca, a clerk at Dairy Del, said she notices the drinks flying off the shelves on Friday afternoons when the young crowd is getting ready to party.

"There was a time when I thought those drinks weren't going to go anywhere," she said. "Then the idea of mixing them came along."

Red Bull, an Austrian company, controls 58.3 percent of the U.S. market, followed not so closely by Hansen's Beverage Co. of Corona, Calif., with 11.9 percent and Rockstar of Las Vegas at 10.7 percent, said John Sicher, editor and publisher of Beverage Digest, which tracks the industry.

"It's not only for athletes and students but for partyers as well," said Jason May, executive vice president of marketing for Rockstar.

The top five drinks account for 89.1 percent of the crowded energy drink market, leaving dozens of other companies to fight for the rest, according to Sicher's data.

First sold mainly in skinny eight-ounce cans, several brands are now available in 16-ounce cans and four-packs. The eight-ounce cans usually sell for about $2.

But some nutrition experts and doctors say consumers should find other less-risky ways to get the lift the drinks provide. Authorities in France, Sweden and Denmark have all warned that Red Bull can cause serious health problems.

Dr. Keith Roache, an internist at New York-Presbyterian Hospital, said runners, cyclists and other athletes have routinely used caffeine before performing but added, "The concern with the energy drinks is that it's a lot of caffeine and a lot of sugar."

A can of Red Bull contains 80 milligrams of caffeine, about the same as a cup of coffee, he said.

"Caffeine has a lot of effects on people and combining caffeine and exercise isn't always a good idea," he said.

He said the drinks increase the amount of heat in the body, posing the danger of heat stroke in some people, particularly on a warm day. The drinks are also diuretics, meaning they can lead to dehydration, he said.

Mixing the drinks with booze is also dangerous, according to David Pearson, a researcher at Ball State University in Muncie, Ind.

The combination of stimulants in some energy drinks and the depressants in the alcohol could cause cardiopulmonary or cardiovascular failures, said Pearson, a researcher in the Human Performance Laboratory.

"I think we are going down the same road as when people drink alcohol and ingest ecstasy and other types of designer drugs," he said in a news release. "Some people physically cannot take the combination."

May of Rockstar said Red Bull and vodka is one of the most popular nightclub drinks in the country.

"If people get drunk that's their issue, that's their problem, that's their irresponsibility," May said.

"I don't encourage that. None of the energy drink companies encourage that."