Prescription to Kill

A woman walks into a pharmacy and asks the pharmacist for some arsenic.

The pharmacist asks "What for?".

The woman says "I want to kill my husband".

The pharmacist says "Sorry, I can't do that".

The woman reaches into her purse and pulls out a photo of her husband in bed with the pharmacist's wife and hands it to the pharmacist.

He then says "You didn't tell me you had a prescription...".