A recent study found that mixed drinks made with diet soda can increase blood alcohol content by 18% over regular sodas.
According to the 2013 study, the stomach processes sugar with regular soda as if it were food. With diet soda, the stomach doesn't recognize anything to digest, and the concoction is sent directly to the small intesting.
Time quoted one of the lead authors as noting, "One of the key things we found was that even though BAC peaked 18% higher in the diet condition, [participants] didn’t feel any more intoxicated and they didn’t feel any different as to how willing they were to drive a car."