Woman walks into a drugstore, goes up to the pharmacist and says, "I'd like to buy some cyanide."
Pharmacist says, "Why?"
"I want to kill my husband."
"Are you nuts? You'll go to jail and I'll lose my license!"

She takes something out of her purse and hands it to the pharmacist. It's a picture of her husband in bed with the pharmacist's wife.

The pharmacist looks at it and says, "Oh! You have a prescription!"