Prescription for cyanide

A lady walks into a drug store and tells the pharmacist she needs some cyanide.
The pharmacist said, "Why in the world do you need cyanide?"
The lady then explained she needed it to poison her husband.
The pharmacist's eyes got big and he said, "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 and all kinds of bad things will happen! Absolutely not, you can NOT have any cyanide!"
Then 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, you didn't tell me you had a prescription."