Energy drinks are occasionally used as mixers with alcohol. This combination carries a number of potential dangers:
Since energy drinks are stimulants and alcohol is a depressant, the combination of effects may be dangerous. The stimulant effects of caffeine can mask how intoxicated you are and prevent you from realizing how much alcohol you have consumed. Fatigue is one of the ways the body normally tells someone that they've had enough to drink. The stimulant effect of caffeine can make a person feel less tired, thus, give them the impression they are not impaired. No matter how alert you feel, your blood alcohol concentration (BAC) is the same as it would be without the energy drink. In fact, research has found that people drink more and have higher BACs when they combine alcohol and caffeine.
Additionally, both energy drinks and alcohol are diuretics (cause water expulsion or loss). Dehydration can hinder your body's ability to metabolize alcohol and will increase the toxicity, and therefore the hangover, the next day.