Fungus Up is a soda in Brain Drain. It is the cure for the mind swapping caused by Iggy Koopa's Synapse Switcher. Once someone who had their mind swapped drinks it, they return to their original bodies. It is first mentioned on a radio broadcast from WMUSH, in which it is advertised as a drink that "springs up overnight" and makes its consumer feel rotten. This makes Luigi want a drink, and he attempts to take a sip from the soda machine. However, the machine spontaneously breaks, forcing Luigi to abandon his drink. After the machine is fixed, Luigi attempts to take another sip, but he is called away by everyone panicking about the mind swapping. If he still tries to sneak in a sip, he instead finds his Tape containing his favorite song. The reader also has the opportunity to solve a puzzle in which Fungus Up, Morel Moxie, and Lichen Cola are featured. Three cups are featured, each with a soda's logo; there are three ounces of soda in the Morel Moxie glass, twice as much Lichen Cola as Morel Moxie, and three times as much Lichen Cola as Fungus Up; and the reader must find out how many ounces of Fungus Up there is and apply that number to a code ("UVQRU DTCKP UYKVEJ") by reversing its position in the alphabet. As the cup contains two ounces of soda, the code reads "STOPS BRAIN SWITCH," letting the reader know about its curing properties.
A can of Fungus Up later appears at WMUSH, though it can be obtained in two different circumstances. If Luigi (in the body of a Koopa Troopa) uses the studio's computer, Mario storms the studio. Iggy boasts that he can never reach the cure while motioning to a can of Fungus Up and a can of Morel Moxie. As Luigi himself does not know which one is the cure, he grabs them both and runs outside to meet Mario. He must then make the choice as to which soda he should drink (the correct choice being Fungus Up, naturally). If Luigi uses the television, Iggy walks away to watch his music tape. Luigi believes that the tape's song contains a hidden meaning. If he has his Tape with him, he can listen to it and crack its code (again by changing the letters' positions according to a number), telling him to press the Synapse Switcher's yellow button. Doing so dispenses a can of Fungus Up, letting Luigi know that it is the cure. He grabs enough cans for him and his friends and promptly brings them to him. Regardless of which path is chosen, Mario, Luigi, Princess Peach, Toad, and the Mushroom King all drink some Fungus Up and return to their bodies. Iggy, who ends up trapped in a Wiggler's body, tries to drink some Fungus Up himself, but the group stops him from taking a sip. They then plan to pour Fungus Up into the Mushroom Kingdom's water supply, thereby curing everyone that was affected by the mind swap.