WARNING: The following contains spoilers for One Piece Chapter #1017 by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English now from Viz Media.
Chapter #1017 of One Piece introduces a new side-effect of Chopper's Rumble Ball: "Babyjiji," otherwise known as "Baby Geezer," or "Old Baby Man," as some fans have dubbed it. This is the newest side effect of using the Rumble Ball since the formula was changed following Caesar Clown's recommendation. The drawback of using this new modification past its limit appears to be that it transforms Chopper's body into his most peculiar form yet -- the body of a baby while his mind is that of an old man.
The Rumble Ball was developed by Chopper to either enhance his own Devil Fruit powers or to allow him to use powers he normally wouldn't be able to. The Rumble Balls "temporarily distort the wavelengths" of the Devil Fruit, which is how Chopper has been able to use new powers, but the effect previously only lasted for three minutes. During One Piece's two-year time skip, Chopper worked with Caesar to improve the formula, which now allows him to be able to use the effects of the Rumble Ball for up to 30 minutes.
Rumble Balls could be stacked but the user would lose control over their Devil Fruit powers. If a second Rumble Ball is taken within six hours of the first, the powers are increased again, but at the cost of being able to control them. A third has a similar effect, but even more so, with the user being unable to control who they use their Devil Fruit powers against, be it friend or foe.
If the user is a Zoan Fruit user, the Rumble Ball allows them to achieve several new transformations with various advantages. The most powerful of these transformations is a large monster that has the advantages of all of the other forms but is extremely difficult to control. The only way to stop a rampaging user is to drop them in some water, have them come in contact with Seastone, or use a nullifying ability to cancel out the effects.
Chopper was able to use the powers of the Rumble Ball without actually having to take them after training during those two years. On top of this, by taking one Rumble Ball, he now is able to achieve the same effect as if he took three and is able to control that effect, if only for a short time. In the Onigashima raid, he used a Rumble Ball past the 30-minute limit during his fight with Queen, and Chapter #1017 reveals that apparently, the side-effect reduces him to a baby form with an elderly mind.
However, he plans on returning to the fight with the use of an ultra-regenerative medicine Miyagi has, so there's no telling what kind of side-effect could happen should he take a second Rumble Ball. Could he get even younger and older at the same time? The Straw Hat doctor is in uncharted waters now.