One in four people in their early 20s have done it—mixed the stimulating effects of an energy drink with the buzz-inducing properties of alcohol. While partiers swig and stay out late, health experts worry that alcoholic energy drinks cloud their judgment in two important ways: by making people think they are not as drunk as if they’d only had alcohol, and causing them to crave another round more strongly. These effects could explain why people who add caffeine to their cocktail are at greater risk of being in an accident or making a decision they will later regret (like getting in the car with a drunk driver) than those who stick to straight booze.

When the world’s first energy drink debuted in 1987, it didn’t take long for Red Bull to find its way behind the bar. Bartenders soon started mixing Red Bull, Monster, and Rockstar with vodka, gin, Jagermeister, and hard cider. These caffeine-laced cocktails became so popular, major beverage companies created canned and bottled versions like Four Loko to sell in convenience stores.

But as the popularity of alcoholic energy drinks rose, so too did the frequency of emergency-room visits by those who drank them. The rate of visits involving energy drinks in general doubled from 10,000 in 2007 to 20,000 in 2011, and about 2,600 of the visits in 2011 had to do with alcoholic energy drinks. That uncomfortable spike prompted the FDA to ban premixed alcoholic energy drinks including Four Loko—which contained 156 mg of caffeine and 12 percent alcohol, or the equivalent of four beers and a cup of coffee—in 2010.

Today, these drinks still flow freely in bars and restaurants—like at TGI Fridays, where the “Diddy Up” cocktail comes with Ciroc vodka, ruby red grapefruit, Red Bull and fresh-squeezed lime. It was added to the menu in 2010 and “continues to be a favorite for many of our Fridays guests” according to a company spokesperson. Dave and Busters boasts the “Raging Berry Bull” made with vanilla vodka, lemonade, and strawberry ice cubes, plus a can of blueberry-flavored Red Bull. The Black Diamond, a bar in Spokane, Washington, features a drink called “Hell Yeah” made with huckleberry vodka, citrus vodka, cranberry, and Red Bull. Jon Legault, the Black Diamond’s general manager, says he added it recently because “a lot of good drinks involve energy drinks nowadays.”

Kathleen Miller, a sociologist and researcher at University at Buffalo, says these cocktails pose a greater risk than the premixed versions that came in cans and bottles because more people are ordering them from bars than ever bought drinks like Four Loko from a store. She adds that college students have been the focus of most of the research on possible risks.

Cecile Marczinski, a psychological researcher from Northern Kentucky University, says the caffeine in these drinks has the ability to mask intoxication which could make people underestimate how drunk they are and impair their ability to cut themselves off. Subjects in several of her experiments who drank alcoholic energy drinks rated their own drunkenness as lower than subjects with the same blood alcohol content who only had alcohol.

Marczinski also says feeling tired is an important factor in many people’s decision to stop drinking, but that caffeine renders these feelings obsolete. “Since caffeine lasts for six hours, that extends that time when you feel really stimulated and alert and that makes you want to drink more,” she explains. Energy drinks contain between 50 and 500 mg of caffeine, along with additives like guarana and ginseng that also act like stimulants in many people. Sodas, another common mixer, contain about 34 mg to 54 mg of caffeine, but can also heighten intoxication as compared with alcohol alone when used as mixers.

From surveys, researchers do know that people who drink alcoholic energy drinks also consume more alcohol and tend to drink for longer than people who drink just booze. This could mean that heavy drinkers are simply more likely to order a vodka Red Bull, but it could also be a direct result of the masking effect—a question that Marczinski is testing right now in her lab.

The results of another recent experiment might also help explain why people who drink alcoholic energy drinks tend to drink more, and for longer. Rebecca McKetin, a researcher at Australian National University, showed that drinking a vodka-Red Bull created a stronger urge in subjects to keep drinking than having a plain cocktail with the same amount of booze.

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.