The prescription

A woman walks into a pharmacy asks the pharmacist for some arsenic poison.
He says, "What do you want with arsenic?" She replies, "I want to kill my husband because he cheats on me with another woman". The pharmacist says, "I can't sell you arsenic so you can kill your husband,lady. Not even if he is cheating on you with another woman." So she reaches into her pocket and pulls out a picture of her husband in bed with the pharmacist's wife.
The pharmacist says "Oh, I didn't realize you had a prescription!"