Arsenic

A lady walks into the drug store and asks the druggist for some arsenic.

The druggist asks "Ma'am, what do you want with arsenic?".

The lady says "To kill my husband."

"I can't sell you any for that reason", says the druggist.

The lady then reaches into her purse and pulls out a photo of a man and a woman in a compromising position. The man is her husband and the lady is the druggist's wife.

When she shows it to the druggist, he looks at the photo and says, " Oh, I didn't know you had a prescription!"