But a number of scientists are worried about highly caffeinated beverages like Red Bull, Rockstar, Monster and Full Throttle, which are popular among teenagers and young adults.

The often bizarre combination of ingredients in these drinks prompted three researchers from the University of Texas Health Science Center at Houston and the University of Queensland in Australia to examine what is known — and not known — about the contents of these beverages, which are sold alongside sodas and sport drinks in supermarkets, drugstores and highway rest stops.

Their review of all the studies in English in the scientific literature, published in November in The Mayo Clinic Proceedings, led them to question both the effectiveness and safety of energy drinks.

Long-Term Effects Unclear

The researchers noted that the drinks contain high levels of caffeine and warned that certain susceptible people risk dangerous, even life-threatening, effects on blood pressure, heart rate and brain function.

The authors noted that “four documented cases of caffeine-associated death have been reported, as well as five separate cases of seizures associated with consumption of energy/power drinks.” Additional reports include an otherwise healthy 28-year-old man who suffered a cardiac arrest after a day of motocross racing; a healthy 18-year-old man who died playing basketball after drinking two cans of Red Bull; and four cases of mania experienced by individuals known to have bipolar disorder.

Using an abbreviation for energy beverages, Dr. John P. Higgins and co-authors wrote in the Mayo journal that because “teens and young adults, both athletes and nonathletes, are consuming E.B.’s at an alarming rate, we need to determine whether long-term use of E.B.’s by this population will translate into deleterious effects later.”

His co-author Troy D. Tuttle, an exercise physiologist at the Houston university, said in an interview: “Almost all the studies done on energy drinks have involved small sample sizes of young, healthy individuals in whom you’re unlikely to see short-term ill effects.

“But what about the long term? What about liver and cardiovascular disease, insulin resistance and diabetes? We could speculate about a lot of possible problems, but we just don’t know.”

He urged the Food and Drug Administration to “step in and regulate this market,” which currently has few restrictions on the kinds and amounts of ingredients and the claims that are made about them. Manufacturers have labeled the beverages “dietary supplements,” which absolves them of the federal regulations that govern sodas and juices and allows producers to make “structure and function” claims, like “Enhances athletic performance” and “Increases caloric burn and mental sharpness.”

As Mr. Tuttle described the marketing strategy for energy drinks, “the companies have taken a cup of coffee — or two or more cups of coffee, added a lot of hip-sounding stuff and marketed it with a hot, modern, trendy push for young people who want to look cool walking around with a can of Red Bull.

“Anyone can buy these drinks, even 11- and 12-year-old kids.”

In an e-mailed statement, the American Beverage Association said, “Most mainstream energy drink brands voluntarily put statements on their containers, including advisories about use by people sensitive to caffeine.” Also, the organization said many of its members voluntarily list the amount of caffeine on their product labels or have provided caffeine content information through their Web sites and consumer hot lines.

Kevin A. Clauson, a doctor of pharmacy at Nova Southeastern University in West Palm Beach, Fla., who had previously reviewed safety issues surrounding energy drinks said that his main concerns were “the amount of caffeine, which can be injurious particularly to people with a pre-existing cardiovascular abnormality” and “the effects of these drinks when they are combined with alcohol, which can have disastrous consequences.”

After several states made moves to ban Four Loko, it was reformulated to remove the caffeine and two other ingredients, guarana and taurine, but Dr. Clauson said that was “unlikely to have a substantial impact” on young people, who will continue to combine alcohol with energy drinks. The caffeine and caffeinelike ingredients in these drinks can mask the perception of inebriation — and that can increase the risk of drunken driving or other dangerous behaviors.

Mr. Tuttle, who works with sports teams, is concerned about the effects of energy drinks on athletic performance. “A lot of kids are reaching for energy beverages instead of sport drinks, which unlike the energy drinks are mostly water with a nominal amount of sugar and electrolytes,” he said. “The energy drinks contain a slew of ingredients, most of which are unresearched, especially in combination with one another.”

A Potent Brew

For an athlete engaged in intense exercise, the high doses of sugar in energy drinks can impair absorption of fluids and result in dehydration. A 16-ounce can of an energy drink may contain 13 teaspoons of sugar and the amount of caffeine found in four or more colas. Mr. Tuttle noted that caffeine, which is known to improve muscle action and performance, especially in endurance activities, is banned in many sports competitions. Thus, consuming an energy drink close to an event could disqualify an athlete.

Other ingredients often found in energy drinks include taurine, glucuronolactone, B vitamins, ginseng, guarana, ginkgo biloba and milk thistle. Mr. Tuttle calls guarana particularly worrisome because it acts as a stimulant, like caffeine.

“The B vitamins, which are important enzymes for energy utilization, are added to legitimize the high levels of sugar,” he said. “But the American diet, which is very high in protein, already has plenty of B vitamins. These drinks are a kind of sensory overload for the body, with too much stuff coming in at the same time.”

Adding alcohol to the mix, as some consumers were doing even before drinks like Four Loko came along, can be a recipe for disaster. Under the stimulation of energy drinks, people may think they are sober when they are not. Such was the fate of Donte’ Stallworth, a wide receiver for the Cleveland Browns who killed a pedestrian with his car in March 2009 after drinking multiple shots of tequila and a Red Bull. Mr. Stallworth said he did not feel intoxicated at the time of the accident.

“Caffeine is being treated as a flavoring agent, not a drug,” Dr. Clauson said. “The average healthy person who consumes one serving of an energy drink is unlikely to encounter difficulty.” Those most likely to get into trouble, he said, are “toxic jocks” who overindulge and those with an underlying heart condition.