She demonstrated this “priming effect” by pouring drinks for 75 participants, half with Smirnoff vodka and soda water, and the other half with vodka and Red Bull. Adding Red Bull made participants twice as likely to want to drink more than if they had consumed only alcohol, which McKetin considered a small to medium effect. She did not study whether participants actually would consume more drinks—just whether or not they wanted to.

“This is a really promising line of research,” Miller says. “There's a whole host of reasons why alcoholic energy drinks may be significantly riskier in terms of over-drinking and adverse outcomes. Priming is just one piece of the equation, but it's an important one.”

Not everyone agrees that alcoholic energy drinks are riskier than plain cocktails. Joris Verster, a pharmacologist at Utrecht University, questions the real-world implications of McKetin and Marczinski’s results. “It is never actually tested by these authors if participants will indeed consume more of the beverage,” Verster says. In his own survey of 2,000 students—which was, notably, funded by Red Bull—subjects reported lower overall consumption of alcoholic energy drinks than those with just alcohol over the course of a night when they were drinking only one or the other.

Other researchers who have looked more directly at the consequences of drinking alcoholic energy drinks do find support for the idea that they are riskier, but often can’t prove that the drinks actually caused a hangover to happen, just that those who drink alcoholic energy drinks also tend to have more hangovers.

Megan Patrick, a sociologist at University of Michigan, surveyed 500 students about their drinking habits. The students who drank alcoholic energy drinks reported two to three times more negative consequences—like having a hangover or passing out—when compared with those who stuck to alcohol.

In another study, Patrick found that caffeine and alcohol don’t even need to be in the same glass to show an effect. Students who drank energy drinks and alcohol in the same day, but not at the same time, were still at a higher risk for negative consequences than those who did not have an energy drink all day. On those days when they did have energy drinks, students also drank 11 percent more alcohol and drank for five percent longer than on days when they did not.

As experts better identify the impacts of alcoholic energy drinks, the effort to make them safer could take several forms. College campuses might caution students to be careful while enjoying alcoholic energy drinks, or state alcohol beverage control boards could step in to regulate these beverages in bars and restaurants, as the FDA did with Four Loko. “There are plenty of ways to keep alcohol safer,” Marczinski says.

There has been a slight drop in teens who try alcoholic energy drinks since Four Loko was outlawed, according to Lloyd Johnston, a sociologist at University of Michigan who leads an ongoing study of drug use among adolescents. Even so, the latest results from 2013 show that one in four high school seniors has tried these drinks in the past year. Once they reach college, these teens could benefit from what researchers have learned from testing the classes that came before them—namely, that ordering a vodka cranberry instead of a vodka Red Bull might be a smarter choice.

We want to hear what you think about this article. Submit a letter to the editor or write to letters@theatlantic.com.