The owner of a drug store walks in to find a guy leaning heavily against a wall. 

The owner asks the clerk "What's with that guy over there by the wall?" 

The clerk replies, "Well, he came in here this morning to get something for his cough. I couldn't find the cough syrup, so I gave him an entire bottle of laxative."

The owner screams, "You goober!  You can't treat a cough with a bottle of laxatives!"

The clerk calmly replies, "Of course you can! Look at him; he's afraid to cough!"