Mixing alcohol with energy drinks is a popular but dangerous habit among U.S. college students, according to new research that found those who combine the two tend to drink more, take more risks and are more likely to get hurt while drinking.

The research, by investigators at Wake Forest University School of Medicine in North Carolina, found students who mix energy drinks with alcohol were twice as likely to be injured during a bout of drinking, to need medical attention or to ride with a driver who was drunk.

'So you're drunk. But you just don't know that you're drunk.'—Dr. Mary Claire O'Brien, researcher

They were also twice as likely to take advantage of someone sexually and nearly twice as likely to be taken advantage of sexually by someone else. The researchers believe the problem is the high caffeine levels in the energy drinks mask the effects of excess alcohol — the stumbling, slurred speech or sleepiness that signal intoxication.

"What I would describe it as is a person for whom the symptoms of drunkenness are reduced, but the drunkenness is not," lead author Dr. Mary Claire O'Brien said in an interview. "So you're drunk. But you just don't know that you're drunk."

O'Brien presented the findings of the study Sunday at the annual meeting of the American Public Health Association in Washington, D.C.

Mixing energy drinks with alcohol is a popular phenomenon, with websites devoted to rating the effectiveness of various combos, said O'Brien, a professor of emergency medicine and public health sciences.

The beverage industry has twigged to the potential, producing pre-mixed versions of popular energy drinks which sell in some locations for less than the non-spiked original, according to a report on energy drink cocktails published in August by the Marin Institute, a California-based alcohol industry watchdog.

O'Brien's study — one of the first to look at the implications of this trend — is based on an internet survey of 4,271 students from 10 U.S. universities. Randomly selected students were invited by e-mail to take part in the survey and were paid a token sum for answering roughly 300 questions on health risk behaviours that focused heavily on alcohol use.

Caffeine functions as override, researchers suggest

Twenty-four per cent of participants reported imbibing energy drinks laced with alcohol in the previous 30 days. Consumption of the combo was more common among students who were male, white, athletes, fraternity member or pledges, and students who were older.

The caffeine in the energy drinks — some contain three times as much as a regular-sized cup of coffee — seems to work as an override. It appears to trick the brains of people who are drinking into thinking they are much less impaired than they actually are.

Dr. Karen Leslie, a pediatrician with the substance abuse program of the division of adolescent medicine at Toronto's Hospital for Sick Children, said the combination exacerbates the already dangerous pattern of binge drinking favoured by teenagers — drinking quickly and to excess.

"So it's entirely not surprising that if young people are taking in more alcohol because they're not noticing the effects of it earlier on because of the caffeine, these are not surprising things at all," Leslie said.