Drinks like Four Loko were banned last year after many college students landed in the hospital.
Researchers at Northern Kentucky University compared two groups: one drank vodka and soda, the other drank vodka and Redbull. They found those who had the Redbull cocktails didn't realize they were as drunk as they actually were.
They say that could lead to over-drinking or driving while impaired.
The results were published in the journal "Alcoholism".