(The New York Times News Service) -- Mixing alcohol and energy drinks such as Red Bull have become increasingly popular, but the combination can result in a disconnect between perception and reality, Brazilian researchers report.

Although drinkers said they felt less tired and had a heightened sensation of pleasure after quaffing a Red Bull, in actual fact their abilities were still significantly impaired.

"This is just the combination that might lead to very bad judgments," says Dr. David L. Katz, an associate professor of public health and director of the Prevention Research Center at the Yale University School of Medicine. "Feeling less intoxicated and more alert, one might get behind the wheel of a car. But with impaired coordination and reaction time, that decision might spell disaster."

The study, by researchers at the Federal University of Sao Paulo, appears in the April issue of Alcoholism: Clinical and Experimental Research.

The effects of the Red Bull and alcohol mix might be due to the ingredients in the drink. Red Bull contains caffeine plus taurine, an amino acid that plays a significant role in brain and retinal development, and may enhance the effects of caffeine. The drink also includes glucuronolactone, which is supposed to stimulate the basal metabolic rate.

"Popular reports suggest that the use of energy drinks might reduce the intensity of the depressant effects of alcohol," says lead researcher Maria Lucia O. Souza-Formigoni, an associate professor and psychobiologist. "However, there was little scientific evidence to support this hypothesis."

In their study, Souza-Formigoni's team assigned 26 young, healthy volunteers to two groups that received 0.6 or 1.0 g/kg of alcohol (vodka) respectively. All the volunteers completed three experimental sessions. These included alcohol alone, Red Bull alone or alcohol plus the energy drink.

Alcohol alone produced higher levels of weakness, difficulty in walking and increase of muscular tension, Souza-Formigoni says.

Alcohol plus Red Bull prevented the sensation of headache induced by alcohol, but like alcohol, it also caused sensations of fatigue, dizziness and weakness, as well as altered sight, walk, hearing and speech when compared to the ingestion of energy drink alone.

In addition, the sensation of general well-being was smaller after mixing alcohol and Red Bull than with alcohol alone.

"The ingestion of one dose of energy drink was not enough to significantly reduce most of the objectively measured effects of alcohol in tests of motor coordination and reaction time, nor to reduce the breath-alcohol concentration," Souza-Formigoni says.

She adds, however, that participants did experience "a reduction in the subjective sensation of intoxication."

In other words, the combination of Red Bull and alcohol doesn't reduce the effects of alcohol -- it only makes you think it does.

"People should be warned that in spite of the sensation of reduced effects, they are still affected by alcohol, and their motor coordination is impaired," Souza-Formigoni says. "They shouldn't drive or do any activity for which motor coordination is required."

The makers of Red Bull don't recommend mixing the product with alcohol. "Red Bull is a nonalcoholic energy drink that vitalizes body and mind. It improves performance and reaction time in moments of need," says Red Bull spokeswoman Patrice Radden. "Alcohol makes you tired, and therefore can impair the positive functional effects of Red Bull. This is why we do not promote Red Bull to be mixed with alcohol."

Katz agrees, and says he remains skeptical as to whether drinks like Red Bull can enhance performance in any situation. "But one thing is perfectly clear, they are no substitute for a designated driver," he says.

"The public health community, along with the makers and distributors of energy drinks, will need to spread the message that impairment from alcohol may feel different under the influence of an energy drink, but still poses all of the same threats."

Copyright 2006 The New York Times News Service. All rights reserved.