Greetings. In a study the average lethal dose for Xanax has been found as 22 mg. But death can occur at 10 mg single dose. With alcohol, there is further central nervous system depression and the lethal dose can be far less.Though for it (combination and alcohol)data are not available.Please read this study; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1884537/. Please feel free for your follow up questions.ith alcohol, there is further central nervous system depression and the lethal dose can be far less. Though for it (combination and alcohol)data are not available. Please read this study; http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1884537/. Please feel free for your follow up questions.