On day nine you'll deal with a junkie near Candy's electronics store. Shortly after you'll receive a call about another at the church, which turns out to be Father Mushroom.
You'll be asked to go get his medication from Krank's drugstore. When you return to Father Mushroom you'll have two options - Slap him, or check the package.
Select the check the package and you'll find the medication is suppositories. Pop one in the Father to earn yourself this unfortunate achievement.