McKetin and her co-author, Alice Coen, rounded up 75 volunteers, aged 18 to 30, and asked them to fill out a questionnaire that rated their urge to drink. Then the volunteers were randomly assigned to receive either an energy drink combined with fruit juice and two shots of vodka or soda water and fruit juice mixed with two shots of vodka.

Questionnaires filled out 20 minutes after the cocktails were downed showed that volunteers who had consumed the combination of alcohol and energy drinks had a greater urge to keep drinking.

What’s intriguing about the new study is that it focused on the early, or “priming,” effects of caffeine and alcohol, said Dr. Larissa Mooney, an addiction psychiatrist and an assistant professor of psychiatry at University of California, Los Angeles. “It shows that something is happening to trigger a desire to want more, and that that happens pretty early on, after the first drink is consumed.”

While the increase in the urge to drink was moderate, “it adds to the larger conversation about alcohol and energy drinks,” said David Jernigan, an associate professor at the Johns Hopkins Bloomberg School of Public Health. “Particularly for young people, mixing alcohol with high levels of caffeine, from all we can tell, may lead to a greater likelihood of bad things happening.”

Dr. Charles O’Brien, a professor of psychiatry at the University of Pennsylvania and director of the Center for Studies in Addiction, agreed. “The real risk of this combination is that people feel they aren’t drunk when they are,” he said. “That’s because caffeine wards off the sedation that normally comes when you drink.”