Rumble Ball

The Rumble Ball (ランブルボール) is a yellow jaw breaker-like drug developed by Tony Tony Chopper through research (and a bit of an accident, according to the anime).

Usage
The "Rumble Ball", when eaten, gives Zoan-type Devil Fruit users additional transformations for three minutes by "distorting the wavelengths of transformation". Before the timeskip, Chopper uses it to gain 4 more transformations. Early on, Chopper tends to say "Rumble" (ランブル) before consuming the drug.

It is revealed, post-timeskip, that he only needs one for Monster Point, which he originally had no control over.

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.

However, after training for two years, Chopper seems to be able to control the transformation now and can stay in it for three minutes after eating just one Rumble Ball.

Trivia

 * According to an SBS, if Chopper eats four Rumble Balls, the author goes wild. In this state, Chopper returns to his Brain Point, but with extreme facial changes: his face becomes rectangular with his cheeks jutting out, his nose growing big, with a toothbrush mustache underneath, and thick lips. He also grows human ears, and his hat turns upside down, and merges with his forehead, with the "X" in the center.
 * According to another SBS, if Oda ate three Rumble Balls, then he would grow out to be a giant.
 * In the anime only, during the first round of the Davy Back Fight's second 3 coin game Chopper took a second Rumble Ball within a matter of minutes of his first wearing off and showed no side effects. This is due to the manga has yet to reveal such side effects when the filler episode came out.
 * The Rumble Ball's name and appearance resembles real-life rum balls, a truffle-like confectionery.

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