From the Super Mario Wiki, the Mario encyclopedia
Crabbie Grass is an item in Mario & Luigi: Superstar Saga and the remake which is required to cure Mario's Bean Fever, a disease in which the victim turns into a bean if it is not cured within three days. Crabbie Grass is a small, bitter-tasting herb which is the only known cure for Bean Fever. Once Mario contracts the disease after eating an Invincishroom (which actually turned out to be a Poison Mushroom in the game's remake), it is up to Luigi to go to Guffawha Ruins and retrieve the Crabbie Grass, which is located at the very end of the temple. Luigi only finds one piece of this herbal medicine, but it is still enough to cure Mario.
In Mario & Luigi: Superstar Saga + Bowser's Minions, the Crabbie Grass is red instead of green.
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