A nice, calm and respectable lady went into the pharmacy, walked right up to the pharmacist, looked straight into his eyes, and said, "I would like to buy some cyanide."
The pharmacist asked, "Why in the world do you need cyanide?"
The lady replied, "I need it to poison my husband."
The pharmacist's eyes got big and he exclaimed, "Lord have mercy!
I can't give you cyanide to kill your husband! That's against the law!
I'll lose my license! They'll throw both of us in jail! All kinds of bad things will happen. Absolutely not! You CANNOT have any cyanide!"
The lady reached into her purse and pulled out a picture of her husband in bed with the pharmacist's wife.
The pharmacist looked at the picture and replied, "Well now, that's different. You didn't tell me you had a prescription."

I have heard that one before, but my version is the husband buying it for the ugly wife

I think i'd like your version better