The Rumble Ball is a yellow jaw breaker-like drug developed by Tony Tony Chopper through research.
Main Article: Human-Human Fruit
As demonstrated and announced by Chopper, the Rumble Ball is not meant to be used casually, as distorting the nature of a Devil Fruit power is dangerous.
The amount of Rumble Balls taken must be closely monitored. If the user takes two within six hours, it causes them to lose control of their transformations. If a third one is taken within the six hours, the user transforms into a huge, virtually invincible monster that combines all the attributes of the other forms provided by the Rumble Ball.