Breath alcohol levels remained higher for the diet soda/alcohol drinkers for the entire three-hour period.
After drinking, the researchers also had the study volunteers perform a test on the computer. Participants who downed the diet drinks performed slightly worse, although they didn't notice any difference in the way they felt or performed.
"They were slower to respond. It was a small difference, but it was statistically significant," said Marczinski.
She said she suspects the alcohol was released from the stomach faster in the diet-drink group because there was no sugar (and no calories) in the stomach to slow down the delivery.
The researchers didn't notice a difference between men and women in this study. But Marczinski said, "women are more inclined to consume alcoholic beverages with diet soda."
The bottom line is that people shouldn't drink on an empty stomach, and they may want to think twice about saving calories by using diet soda mixers, she said.
One expert agreed the findings should encourage caution.
"If you haven't eaten before you start drinking, your blood sugar will go down. And then if you use diet mixers, you're at a much higher risk of being intoxicated," said Samantha Heller, a clinical nutritionist at the NYU Center for Musculoskeletal Care in New York City.
"If you think you're doing a better thing by going with the diet mixer, you need to know you may be doing yourself a disservice if you're getting more drink faster," she added.
And while this study's findings are "a concern," Heller also noted that the study is quite small, and it should be duplicated.
Still, she advised anyone who's been drinking to hang up the car keys. "Don't drink and drive, no matter what. It's just not worth it," she said.
The American Beverage Association took issue with the findings.
"This paper, which looks at only 16 people, does not show that mixing diet soft drinks with alcohol causes increased intoxication," the association said in a statement released Tuesday. "Rather, it simply supports the long known fact that consuming calories -- from any food or beverage -- along with alcohol slows down its impact."