Bartenders say, asking for diet is a popular option.

"People just assume that it's healthier for 'em," says bartender Angela Bartholomew.

But a new study says otherwise.

Researchers at Northern Kentucky University gave participants one of three vodka drinks -- diet, regular or a placebo. They concluded, the diet soda mix made for a more potent drink and resulted in an 18% higher breath alcohol concentration.

But, one doctor says that is not as much as it sounds.

"Certainly 18% difference in the alcohol levels in your body is a really small amount, and it wouldn't surprise me that people couldn't tell the difference," says Dr. Todd Reynolds of Prevea Health.

But, it could mean the difference between being under or over the legal limit of .08.

Doctors say there is a lot more that goes into calculating your blood or breath alcohol content than drinking diet or regular soda, for instance, are you drinking on a full or empty stomach?

"If you have a stomach full of food, you can decrease your alcohol absorption by easily twice as long," says Dr. Reynolds.

Bartholomew says, no matter what you're drinking, bartenders know when you have had too many.

"You can tell when they've reached their limit, and you know when you should cut them off, so I mean, whether it be three drinks with diet soda, or seven drinks with regular soda, it is going to be the same effect," says Bartholomew.

Researchers think the sugar drinks, or regular soda, help slow down the absorption of alcohol into the bloodstream.