Super Sushi Shrinker
From the Super Mario Wiki, the Mario encyclopedia
Super Sushi Shrinker is a creation of famous scientist Dr. T. Garden, who makes it to counteract the effects of his other invention, Super Sushi. Super Sushi Shrinker has a liquid composition and must be injected into someone who has eaten Super Sushi in order for it to take effect and shrink that person.
In The Super Mario Bros. Super Show! episode "Mario Meets Koop-zilla", Dr. T. Garden, as Mario and Koopzilla, who have both ingested Super Sushi, battle, creates Super Sushi Shrinker to shrink both of them back to normal size.
Having been given a large amount of Super Sushi Shrinker, which is placed in a giant needle, Luigi, Toad and Princess Toadstool venture into the ruined city of Sayonara in search of Mario and Koopzilla. Eventually finding the two, Luigi, Toad and Princess Toadstool manage to inject Koopzilla with Super Sushi Shrinker after shoving their large needle into his ankle. After being administered this dosage of Super Sushi Shrinker, Koopzilla is forcibly shrunk back to his normal size and flees to a Warp Zone.
Evidently, Mario is given Super Sushi Shrinker off-screen, as he is shown returned to his normal size later on in the episode.