Rumble Ball

The Rumble Ball (ランブル･ボール, Ranburu Booru) is a yellow jaw breaker-like drug developed by Tony Tony Chopper through research (and a bit of an accident, according to the Anime). It first appeared in Chapter 148 and Episode 88.

Usage
The "Rumble Ball", when eaten, gives Zoan-type Devil Fruit users additional transformations for three minutes by "distorting the wavelengths of transformation". For Chopper, it comes with 4 more transformations. For details on the various combination forms, see Hito Hito no Mi.

Side Effects
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. This monster form has no known time limit, but is dangerous to friend, foe and the user, not only because the user loses their mind and attacks everything in sight, but also to maintain the transformation requires lethal amounts of energy, which could effectively lead the user to die from fatigue. The only known way to stop this is to use any Devil Fruit nullifying methods such as submerging the user in a body of water or Kairoseki (which "emits the same energy as the sea") to disable the user's powers.