A lady walks into a chemist's and asks for some arsenic. The chemist asks, "What do you want with arsenic?"
The lady says, "To kill my husband."
"I can't sell you any for that reason," the chemist replies.
The lady pulls out a photo of a man and a woman in a compromising position and shows it to the chemist. The photo shows her husband with the chemist's wife.
He looks at the photo and says, "Oh, I didn't know you had a prescription!"