Arsenic

A lady walks into the drugstore and asks the pharmacist for some arsenic.

"Ma'am, what do you want with arsenic?" "To kill my husband."

"I can't sell you arsenic to kill a person!"

The lady lays down a photo of a man and a woman in a compromising position.

The man is her husband and the woman is the pharmacist's wife.

He takes the photo, and nods. "I didn't realize you had a prescription!"