Pharmacist's Wife

A woman walks into a pharmacy and asks the pharmacist for some arcenic poison. He says "What do you want with arsenic?" The wife replies "I want to kill my husband for cheating on me with another women" The pharmacist says "I cant sell you arsenic so can kill your husband lady, even if he is cheating on you with another women. So the wife reaches into her pocket and pulls out a picture of her husband and the pharmacist's wife. The pharmacist replies "Oh, I didnt know that you had a prescription."