In 2010, the Food and Drug Administration sent letters out to beverage makers warning that their caffeinated, alcoholic drinks were “unsafe.” The federal admonishment followed an exceptional string of reports that college kids were getting black-out drunk and severe alcohol poisoning after consuming them. Mixing alcohol and high levels of caffeine is a dangerous combination, the FDA and health experts cautioned; the drinks amp people up while dousing their ability to sense their own intoxication, leading to more drinking and riskier behavior.

But according to new research, highly caffeinated beverages can be linked to serious trouble.

In a six-year study following 1,000 college students, researchers found that the more non-alcoholic energy drinks a person reported throwing back, the more likely they were to drive drunk. The finding squares with past studies that have linked alcoholic energy drinks to such dangerous behaviors. However, the study, published Tuesday in Alcoholism: Clinical and Experimental Research, is the first to decouple the bad effects of alcohol from those of the energy drinks alone.

“[The] results shed light on the complexity of the relationship between [energy drink] consumption patterns and an important public health problem: drunk driving,” noted the authors of the study, led by public health researcher Amelia Arria of the University of Maryland.

How exactly a non-alcoholic beverage correlates to more drunk driving—which obviously involves alcohol—is not yet clear from the survey data, the authors note. However, the researchers speculate that drinking energy drinks prior to or alongside alcoholic ones may allow a drinker to become “wide-awake drunk,” paving the way for more drinking and risky behavior such as drunk driving. It’s also possible that the students were consuming energy drinks after drinking in order to nurse a hangover. In that case, energy drink consumption would still be a useful flag for targeted drunk driving prevention campaigns, the authors note.

The authors also suggest psychosocial factors that may explain the data; the type of people who drink energy drinks may be the type already prone to driving drunk—or at least admitting to it in a study. Advertisements and marketing campaigns for energy drinks tend to zoom in on people who are “characterized by an idealized notion of an exciting, active lifestyle with a proudly carefree and undaunted attitude of ‘living for the moment,’” the authors wrote. “In that case, it would be plausible that individuals who identify with such a prototype might also be at risk for drunk driving because they tend to dismiss any potential for harm.” And then there’s the chance that “willingness to admit to or even embrace a stigmatized behavior (i.e., drunk driving) might be overrepresented among the target-market of [energy drink] products.”

The researchers call for followup studies to try to tease such factors apart.

In their study, the authors tried to eliminate some other potentially complicating factors; they took into account family history of alcohol use, participants’ tendencies for risky behavior, depression, and use of other caffeinated beverages, such as coffee. The 1,000 students were followed for six years with annual surveys that probed their alcohol use, energy drink use, and drunk driving frequency, among other things.

By the end, when the students’ most common age was 23, nearly all reported that they drank alcohol at least once in the previous year, 25 percent reported driving drunk, and 57 percent reported having at least on energy drink. Among those energy drink drinkers, 56 percent said they drank them both alone and mixed with alcohol, 15 percent said they only drank them if they were mixed with alcohol, and 27 percent said they took their energy drinks neat and drank alcohol separately.

In their analysis, the researchers found that drunk driving reports were strongly linked with more energy drink use—both with and without alcohol—as were, unsurprisingly, reports of more and frequent alcohol use.

Alcoholism: Clinical and Experimental Research, 2016. DOI: 10.1111/acer.13229 (About DOIs).