Rumble Ball

The Rumble Ball is a yellow jaw breaker-like drug developed by Tony Tony Chopper through research.

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 four more transformations.

Early on, Chopper tended to say "Rumble" (Ranburu) before consuming the drug.

After the timeskip, Chopper only needs one for Monster Point, which he originally had no control over. Through his training, he gains the ability to change between six other forms without consuming the drug.

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 user alike, 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 Seastone (which "emits the same energy as the sea") to disable the user's powers.

However, after training for two years, Chopper is now able to control the transformation without the use of Rumble Ball. He only uses it when turning to Monster point, which he can now control, but when Franky (in Chopper's body) eats a Rumble Ball, he wasn't able to control the Monster Point and rampaged attacking friend and foe alike.

Trivia

 * According to the 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.
 * While the manga never depicts it, the anime shows Chopper discovering the Rumble Ball by accident after experimenting.
 * According to another SBS, if Oda ate three Rumble Balls, then he would grow out to be a giant.
 * In the anime, 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 having 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.
 * Through his development of the Rumble Ball, Chopper is the first Devil Fruit user to artificially augment his Devil Fruit powers. However Chopper is not the only one to do so, as Caesar Clown (by his own admission) used his chemical and scientific expertise to artificially augment the Gasu Gasu no Mi to grant himself new offensive abilities.