Poison: Occasionally deals 1/16 MAX HP damage.

Poison [!Black] - 99% Hit Rate.

Black Mage | Red Mage

World 1, Buyable at Karnak, Crescent for 290 Gil

Black Mage | Blue Mage | Freelancer | Mime | Red Mage | Summoner | Time Mage

World 3, Wonder Rod

Poison Sword [!Spellblade] - Always Procs.

Mystic Knight

World 1, Buyable at Karnak, Crescent for 290 Gil

Spore [Enemy Spell] - 75% Hit Rate.

Beastmaster

World 1, Pirate Cave - Stroper
World 1, Shifting Sands Desert - Sandboil
World 1, Overworld (Ocean) - Thunder Anemone
World 3, Istory Falls - Coral

Poison Breath [Enemy Spell] - 75% Proc Rate.

Beastmaster

World 2, Drakenvale - Zombie Dragon

Skunk [!Animals] - Always Hits. Also inflicts Blind.

Ranger

Animals. Unlocks at L40.

Poison Mist [!Gaia] - 99% Proc Rate.

Geomancer

Available at L11 for the Pyramids.
Available at L21 for marshlands

Dud Potion [!Mix] - Always Hits.

Chemist

Mix Holy Water + Dark Matter

Poison [!Mix] - 99% Hit Rate.

Chemist

Mix Antidote + Dark Matter

Poison Axe [Weapon] - 67% Proc Rate.

Berserker | Freelancer

World 2, Buyable at Surgate for 9600 Gil
World 2, Barrier Tower - Wall Knight (Rare Steal)
World 3, Pyramid of Moore - BioSoldier (Rare Drop, only from version summoned by Zephyrus)