I know saltpeter is just a rumor. It doesnt actually work and could have major side effects. I am looking to play a joke on a friend and could use some help.

Does anyone know what I could use to slip into my buddies food or drink that will keep him from getting an erection for a few hours? He has a hot date and I want to play a joke on him. I dont want to hurt him or for him to get any side effects. Any help would be appreciated. Thanks in advance.