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

The lady then reaches into her purse and pulls out a photo of her husband in bed with the pharmacist's wife. 

He looks at the photo and says "Oh...........I didn't know you had a prescription!"