There’s nothing wrong with having an occasional cocktail, but if you’re putting diet drinks into the mix in an effort to reduce calories, don't. When people use diet drinks in their mixed drinks rather than sugar-sweetened mixers, blood alcohol levels rise even higher than normal. Of most concern is that people tended not to notice the effect, even when their breath alcohol had reached levels at which it was illegal to get behind the wheel.

Sixteen volunteers were given three different drinks in three separate sessions. In one they drank vodka mixed with a regular soft drink. In another they had precisely the same amount of vodka mixed with the diet version of the same soda as before. And in the last session, participants had a placebo beverage with no alcohol.

The research team measured each person's breath alcohol concentration. The participants also rated how intoxicated they felt, how willing to drive they’d be, and how fatigued, sedated, and impaired they felt overall and completed simple tests to measure their reaction times.

Participants’ breath alcohol concentration readings were significantly higher if they drank alcohol mixed with diet mixers rather than alcohol and sugar-sweetened drink combinations. Scores on reaction time tests were considerably lower for those who drank the alcohol-diet drink combination. And because the two groups did not differ in how intoxicated or otherwise impaired they said they felt, the more intoxicated alcohol-diet drink group might be as likely to get behind the wheel.

What is it about alcohol mixed with diet drinks that makes them more intoxicating? It is not yet completely clear, but it may be because the sugar in regular drinks acts like a "food," slowing down the rate that the alcohol is processed by the stomach and absorbed by the body. Previous studies have suggested that the stomach moves its contents through to the bloodstream faster when alcohol is mixed with diet drinks, as compared to regular drinks. The result is a higher blood alcohol level.

The important finding in this study is that people’s blood alcohol content was above the legal limit of 0.08% when they had consumed the diet drink mixture. It’s particularly concerning since these people didn’t feel more intoxicated. As author Cecile A. Marczinski points out in a press release, our choices to get behind the wheel, "or engage in any other risky behavior, often depend on how people feel, rather than some objective measurement of impairment."

Though many of us try to watch our weight these days, mixing alcohol with diet drinks may not be the way to do it. As always, if you’re drinking any kind of alcoholic drink, whether diet or regular, have a designated driver or take a taxi home after a social event— it’s not worth the risk.

The study was carried out by a team at Northern Kentucky University, and published in the journal, Alcoholism: Clinical & Experimental Research.