G.O.B.: They’re pills that create a sort of temporary forgettingness. So if somebody finds out how you do a trick, you just give ’em one of these, and they forget the whole thing. It’s a mainstay of the magician’s toolkit, like how clowns always have a rag soaked in ether. From "Forget-Me-Now"
Forget-Me-Nows are a pill G.O.B. makes use of when he needs people to forget an event. G.O.B. gets caught in a roofie circle for several years due to his taking the pill.
Tobias identified them as Flunitrazepam, or Rohypnol, tablets (also known as roofies), which cause temporary amnesia. Such tablets have been known to be used for a number of illegal and morally reprehensible actions such as date rape and robbery.
G.O.B. ingests a Forget-Me-Now after having a heart-to-heart with Steve Holt. ("Forget-Me-Now")
G.O.B. asks a prison guard "I’ve got a thing of pills in my pocket. I don’t suppose I could convince you to grab me one?" when he is forced against the window of his parent's conjugal trailer. ("Prison Break-In")
Buster supposedly got into G.O.B.'s Forget-Me-Now pills in order to avoid testifying in court. ("Fakin' It")