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The "Rumble Ball", when eaten by a Devil Fruit user, temporarily "distorts the wavelength" of the fruit, granting the user new powers that are not possible otherwise. The Rumble Ball's effect is active from the instant it is swallowed. Originally, the effect of the Rumble Ball only lasted for three minutes, but at some point before reaching Dressrosa, Caesar Clown learned of the Rumble Balls and was able to help Chopper improve their formula with certain ingredients, so that their effects lasted up to thirty minutes.
If a second Rumble Ball is ingested within six hours of the first, the user will gain the additional powers, but won't be able to control them. A third Rumble Ball would cause their Devil Fruit power to go completely out of control, something that is potentially deadly to friends, foes, and user alike.
With mastery, is possible for a Devil Fruit user to freely utilize the additional powers granted by the Rumble Ball without actually taking the drug, and do so at any time and without a time limit. Chopper was able to achieve this after two years of training and research. Additionally, once this level of mastery is achieved, a Devil Fruit user can access the power of three Rumble Balls by taking only one, and control that immense power for a short time as well.
Early on, Chopper tended to say "Rumble" (ランブル Ranburu?) before consuming the drug.
Zoan Devil Fruits
- Further information: Hito Hito no Mi
When consumed by a Zoan-type Devil Fruit user, the Rumble Ball grants them several additional transformations with various advantageous attributes that they don't get from the standard three-point transformations, such as superior strength in their arms or legs, or an enhanced version of their animal characteristics, such as thicker fur that can be used for defense, or larger horns that carry a stronger offense. With three Rumble Balls, the user is transformed into a gigantic, super-strong monster that seems to combine the advantageous attributes of all of their other forms, including the forms granted to them by the Rumble Ball.
With training, these extra transformations can become stronger; after two years of training, Chopper developed his Guard Point from a large ball of defensive fluff to a humongous ball of super-defensive fluff, drastically increasing its size and defensive capabilities. He also developed his Horn Point; after his training, his antlers in this form are larger and thicker than they previously were, albeit less sharp, and he changed its body structure so that he is on two legs instead of down on all fours.
Notably, it is also possible for a Zoan user to develop even more, entirely new Rumble Ball transformations, as seen with Chopper when he developed a new form, Kung Fu Point.
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 very dangerous.
The amount of Rumble Balls taken must be closely monitored. If the user takes a second Rumble Ball within six hours of taking the first, it causes them to lose control of their enhanced powers. In the case of a Zoan-type, they lose control of which form they take when they transform.
If a third one is taken within the six hours, the Zoan 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, yet is dangerous to friend, foe, and user alike, not only because the user loses their mind and attacks everything in sight, but also because maintaining 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 disable the user's powers using any Devil Fruit nullifying methods, such as submerging the user in a body of water or bringing them into contact with Seastone (which "emits the same energy as the sea").
However, after training for two years, Chopper is now able to utilize and control the enhanced powers at all times without needing to take the Rumble Ball at all. However, he still needs it when transforming into the monster form, during which he can now stay in control of himself. Notably, when Franky (in Chopper's body) eats a Rumble Ball, he wasn't able to control the monster form and rampaged attacking friend and foe alike.
After Chopper implemented modifications in his Rumble Ball's formula as recommended by Caesar Clown, he became able to extend the duration of his Monster Point, albeit Caesar warned him of potential risks. Once Monster Point's time limit expires, Chopper gains a completely new form that turns him infant-sized and capable of communication only through antiquated speech.
Anime and Manga Differences
While the manga never depicts it, the anime shows Chopper discovering the Rumble Ball by accident after experimenting with Dr. Kureha's medicine ingredients and trying to hide from her the fact that he was using them by shoving the resulting compound into his mouth.
In the anime, during the first round of the Davy Back Fight's second 3 coin game between the Straw Hat Pirates and the Foxy Pirates, 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.
- It is currently unknown what the Rumble Ball would do to Logia and Paramecia Fruits.
- 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.
- Incidentally, Caesar would later be the one to help Chopper improve the time duration of the Rumble Balls ten fold.
- 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.
- According to another SBS, if Ikue Otani (Chopper's Japanese voice actress) ate three Rumble Balls, then she would grow out to be a giant.
- One Piece Manga and Anime — Vol. 17 Chapter 149 (p. 3-9) and Episode 88, The effects of the Rumble Ball are explained.
- One Piece Manga — Chapter 1014 (p. 6), Caesar helps Chopper improve the duration time of the Rumble Balls.
- One Piece Manga and Anime — Vol. 42 Chapter 407 and Episode 209, The effects of two and three Rumble Balls consumed are explained.
- One Piece Manga — Vol. 67 Chapter 667, Franky lost control after taking one Rumble Ball.
- One Piece Manga — Chapter 1017 (p. 12), Babyjiji Chopper.
- SBS One Piece Manga — Vol. 48, "Very good, sir. → (Picture) When he eats three, his Devil Fruit Powers go wild, but if he eats FOUR Rumble Balls, the author goes wild."
- SBS One Piece Manga — Vol. 58, Fan question: "If you ate 3 rumble balls, what would happen? Please draw it. P.N. Chappy-"