Koro

Koro is a chemical gas weapon invented by Caesar Clown and used by the Beasts Pirates. It's described as a mass slaughter weapon.

Effects
Koro is a poisonous gas that spreads quickly, intoxicating and weakening those that breathe it in. It's effectiveness was enough to surpass the expectations of its own creator, Caesar Clown.

However, despite its strength, its effects are treatable during the initial 48 hours after exposure, with the poison becoming impossible to treat afterwards.

Tony Tony Chopper was also able to develop an antidote for the gas' poison with the help of Caesar Clown.

Neutralization
Ordered by Tony Tony Chopper and Sanji, Caesar created a gas to completely neutralize Koro's effects and named it "Roko". Roko managed to eradicate Koro's residue, but not heal those who ware effected by it.

Trivia

 * "Koro" is derived from the word "Korosu" that mean "Kill".