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. 
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.