She had a prescription

A lady went into a drug store and asked for some cyanide.

The pharmacist asked, "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, "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!

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