For people who actively drink Bacchus, for example, no longer effective painkillers, dental anesthesia, or even surgical anesthesia.
The saddest thing that sometimes it is impossible to predict - what is the use of alcohol will result in trouble as a drink. Much depends on your individual characteristics, the drugs and alcohol.
Classic example of summation effects were shown in the movie "The criminal talent." Heroine acquainted with the men in the restaurant, they drank, the morning wake up completely unlucky philanderers totaly cleared and very patient with his head. Sedation alcohol was superimposed on the effect of clonidine, which is mixed with a swindler and his victims. People quickly "cut off" and in the morning did not remember that happening to them.