Caffeine is a stimulant, meaning it speeds up the functioning of the central nervous system, and it masks the symptoms of alcohol intoxification and prompts you to drink more alcohol than you had intended. Also, if a caffeinated beverage is carbonated, alcohol will enter the bloodstream faster as the effervescence causes the stomach to empty quicker. This is why many people report champagne "going right to their head", the high carbonation allows the alcohol to enter the bloodstream much faster than flat/still beverages.
You may have noticed that you start to become tired, lose coordination, or slur your speech while drinking, which are side effects of alcohol being a depressant; tiredness/poor coordination/slurred speech are signals your body is sending you to slow down your drinking. But if you have a lot of caffeine in your system at the same time, you're far less likely to notice these side effects and are more likely to keep drinking-even past your limit. Keep in mind is that when combining alcohol and caffeine, a person may "feel sober" but they are NOT sober; that feeling is a result of the caffeine masking alcohol's symptoms, but that doesn't mean the alcohol isn't affecting them. Although many popular drinks mix these two substances, it's a good idea to use caffeine and alcohol together sparingly.
What do Northeastern students have to say about mixing alcohol & energy drinks?
Sophomore students from the School of Pharmacy in the Therapeutics with Self-Care Emphasis course made a public service announcement about some facts to consider before you mix these two substances, check out the clip here.