The Rumble Ball condenses various chemicals into a spherical yellow pill, similar to a jawbreaker in appearance. It is easily crushed with a single bite.
Once ingested, the drug immediately "distorts the wavelength" of the user's Devil Fruit-enhanced biochemistry, creating a number of new abilities. In Chopper's case, nearly all of these entail special transformations that emphasize and combine different traits from his three "natural" Zoan forms.
(When consuming the drug, Chopper will frequently say "Rumble" (ランブル Ranburu?), as if designating it a technique in itself.)
Chopper's original Rumble Ball formula—used throughout his life on Drum Island and the Straw Hat Pirates' Paradise adventures—enabled four additional forms that respectively augmented his fur (Guard Point), antlers (Horn Point), hindlimbs (Jumping Point), and forelimbs (Arm Point). It also granted a few augmentations to his "natural" forms, most notably the extra brainpower used in his Brain Point's Scope technique.
This version of the Rumble Ball was effective for only three minutes after ingestion, and properly required six-hour breaks between uses. Disregarding this and ingesting more than one within six hours would inflict dangerous side-effects:
- Two Rumble balls would extend the effects, but completely randomize any control over the transformations.
- Three Rumble Balls would produce a new "Monster Point" with enormous size and strength, but no conscious thought except blind rage. This form was not only dangerous to friend and foe alike, but rapidly drained Chopper's own energy; it would only withdraw with the drug's natural expiration, or with complete Devil Fruit negation (e.g. water-submergence), and in turn leave him paralyzed in Brain Point for several hours.
For these reasons, Chopper more often than not treated the Rumble Ball as a final resort in his early battles.
New World Version
After two years of research and training within the Torino Kingdom, Chopper has mastered his physiology enough to access all four of his "standard" Rumble Ball forms (as well as new ones, e.g. Kung Fu Point) without actually taking the drug. As such, his only remaining use for it in the New World is his "Monster Point"—now accessible with only one Rumble Ball, and without losing any conscious thought.
The latest version of the Rumble Ball (developed with help from Caesar Clown) strengthens Chopper's abilities even further, allowing thirty minutes' worth of transformation. However, entering Monster Point still risks a severe disadvantage, as its expiring will leave Chopper in a shrunken version of Brain Point, largely powerless (and, for unknown reasons, prone to antiquated speech); how long this form lasts before Chopper can safely take another Rumble Ball remains unknown.
Chopper created the first Rumble Ball on Drum Island, by complete accident; in an attempt to hide the fact that he had "borrowed" many of Dr. Kureha's rare medicines to experiment with, he compounded them all together and swallowed the resulting product. Almost immediately, his arm muscles grew to massive size—his first experience with an additional form.
Concludes non-canon section.
Properly developing the Rumble Ball (and mastering its additional transformations) took Chopper five years of research and experimentation, extending through almost his entire tutelage under Dr. Kureha. These experiments sometimes had severe consequences; Chopper's initial discovery of Monster Point, in particular, produced a rampage that nearly destroyed an entire village as well as his own body.
Sea of Survival: Super Rookies Saga
In any case, the Rumble Ball served Chopper well throughout his early battles—against Chessmarimo on Drum Island, Baroque Works on Arabasta, Gedatsu on Skypiea, and the Franky Family on Water 7. Its limitations became near-fatal on Enies Lobby, however, as Chopper was unable to defeat Kumadori within three minutes, forcing him to take a second and eventually a third Rumble Ball. The resulting transformation into Monster Point—Chopper's first-ever deliberate attempt—easily crushed Kumadori, but also threatened several of the other Straw Hats, forcing Franky to knock him into the ocean.
Some time later, the Rumble Ball proved still more inadequate against the Straw Hats' Sabaody enemies, with even Monster Point failing to inflict any serious damage on Sentomaru; ultimately, Chopper survived only by the hand of Bartholomew Kuma, who "vanished" him to the distant Torino Kingdom in the South Blue. Here, Chopper committed himself to researching and training all his abilities—Rumble Ball included—for the next two years, anticipating the Straw Hats' reunion and journey for the New World.
The Final Sea: The New World Saga
After the Straw Hats reunited, Chopper soon demonstrated his new expertise on Fish-Man Island, entering Monster Point with only one Rumble Ball—and all faculties intact. New challenges arose on Punk Hazard, however, as Trafalgar Law's abilities (temporarily) transplanted Franky's mind into Chopper's body; Franky proved disastrous at using the Rumble Ball, immediately succumbing to Monster Point's rage and eventually forcing Luffy to beat him unconscious.
Further improvements on the Rumble Ball came from an ironic source: Caesar Clown, reduced to a prisoner of the Straw Hats following their victories on Punk Hazard. While still unrepentantly cruel, he apparently took genuine interest in Chopper's research, and volunteered a formula to extend the Rumble Ball's duration tenfold.
Anime and Manga Differences
Apart from the aforementioned anime-only origin, the Rumble Ball's side-effects were—prior to the Enies Lobby Arc—unknown to the anime staff, resulting in Chopper's usage being somewhat more casual in anime-original stories. This was most noticeable during the Long Ring Long Land Arc, which featured Chopper (who took no active part in the manga) swallowing two Rumble Balls during the Davy Back Fight's "Hit and Dead Ball" game without any issue.
Translation and Dub Issues
The 4Kids anime has Wapol liken the Rumble Ball's appearance to a fruit rather than candy, possibly to deter connotations with illegal drugs (which often reference candy and other sweets in their street-names); however, it makes no attempt to change the overall steroid-esque effects.
In addition, the 4Kids dub of Episode 88 contains an original segment ("Deer Tales") reviewing all of Chopper's forms, as well as the Rumble Ball's capabilities.
- Thus far, the Rumble Ball's only user—physiologically—has been Chopper himself. It remains unknown what the drug would do to other Zoan users (particularly those whose animal forms lack horns, fur, and/or limbs), Logia users, or Paramecia users.
- In both name and appearance, this drug may have been inspired by the rum ball, 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 tenfold.
- 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.