Poison

Poison is a general term that refers to chemicals that are harmful to organic lifeforms. Poison can come in different forms and phases, and it can have various effects on the victims, depending on its composition. This can range from paralysis to death. Poison can be either created by human chemistry, or secreted naturally from animals or plants as a defense mechanism.

Antidotes
Although poison can be harmful to the body, some of them have a counteracting agent, known as an antidote. They can defuse the poison within the body, saving the victims from death or other harmful effects. However, some poisons may not have an antidote, or some antidotes may be difficult and time-consuming to create.

Poison in One Piece
In One Piece, there are few characters who use poison to fight. They gain a huge advantage in battle, as their opponents are weakened by the toxins and that it is not difficult for one to be poisoned (as opposed to a swordsman who can block sword slashes, or someone like Monkey D. Luffy who takes little damage from blunt attacks like punches), which gives the users more opportunities to strike.

Also, Luffy has developed a high resistance (if not outright immunity) against poison, due to his near death experience against Magellan, and can produce antitoxins to combat the foreign hazard without even noticing it.

Poison-Weapon Users

 * Don Krieg: uses the Deadly Poison Gas Bomb: MH5, which is a poison-gas grenade launcher that creates a large cloud poison, usually lethal to those who breathe it. Krieg claimed that the MH5 is strong enough to annihilate an entire village. Krieg and his men are given gas masks to protect themselves from it. There are no known antidotes.
 * Crocodile: uses a golden-alloy hook with a hidden hook inside that contains scorpion venom. The poison appears to be acidic in large quantities and is concentrated enough to dissolve rocks. Upon being afflicted by the poison, the victim would be paralyzed and would eventually die from cardiac arrest. Nico Robin was carrying a bottle of antidote for this poison.
 * Wanze: uses a giant knife covered in a one-hit sure kill deadly poison in his Ramen Kenpo's final technique: Noodle-Cutting Poisonous Kitchen-Knife.
 * The unnamed pirate crew that massacred the Rumbar Pirates coated their weapons with poison to give themselves an edge in battle. As the Rumbar Pirates' doctor was killed in battle, the crew had no means of saving themselves from death.
 * Duval: uses a harpoon gun with the ammunition being coated with scorpion poison. The poison will kill the victim within three minutes.

Devil Fruit Based Poisons
There are certain Devil Fruit powers that can grant its eaters the ability to create poison.
 * Musshuru: a non-canonical character from Movie 9, eater of the Noko Noko no Mi, which allows him to control and generate poisonous mushroom spores. These spores can be lethal enough to destroy the entire Drum Island, but they are instantly destroyed when they come into contact with fire. An antidote was used by Nico Robin on Monkey D. Luffy.
 * Boa Marigold: eater of the Hebi Hebi no Mi, Model: King Cobra, which allows her to transform into a King Cobra, and allows her to spit venom at her opponents.
 * Magellan: eater of the Doku Doku no Mi, this grants him the ability to create various types of poison, as well as makes him immune to all forms of poison. Each of them have their various effects. Alone, these poisons seem to have an antidote all their own, but if accumulated, curing them could prove difficult, if not impossible.
 * Hydra: a semi-corrosive liquid poison that causes lethal nerve paralysis until the victim dies after 24-hours of pain. The liquid can also be launched in bubbles known as "Poison Blowfish". Fire seems to somewhat neutralize it, while wax and stone are resistant to it.
 * Chloro Ball: a bubble that releases a tear gas poison and immobilizes the victims through severe sneezing. Guards of Impel Down wear gas masks to protect themselves from this.
 * Poison Cloud: a widespread poisonous cloud that slowly robs the victims of their vision, hearing, and strength. Guards of Impel Down wear gas masks to protect themselves from this.
 * Kinjite: Magellan's ultimate poison, it spreads like a disease upon contact with anything, even inorganic materials like wax and rocks. It causes the victims to die instantly once the poison spreads completely through the infected body. It is capable of destroying all of Impel Down.

Wildlife

 * Agehaguera: a giant butterfly that releases toxic pollen when it flaps its wings.
 * Amiudake: a very poisonous mushroom that will kill anyone who eats it in one hour.
 * Daft Green: a species of tree that releases deadly spores.
 * Desert Strawberry: a poisonous spider that strongly resembles a strawberry, killing anyone who eats it after a few hours, and spreads its toxin after, poisoning anyone in the vicinity.
 * Giant Python: a gigantic snake that releases very corrosive venom.
 * Yuda: ferocious and poisonous sea serpents that even Sea Kings do not dare attack.
 * Puzzle Scorpions: very poisonous scorpions that can combine into gigantic centipedes.
 * Hyozo: a blue-ringed octopus merman who can inject deadly venom into opponents.