There are quite a few rules. Unfortunately they are kind of spread out. I will be including references to each rule as I list them, the references will be abbreviated according to this chart:
DMG - Dungeon Master's Guide
PHB - Player's Handbook
MM - Monster Manual
GGtR - Guildmaster's Guide to Ravnica
XGtE - Xanathar's Guide to Everything
Types of Poison
The DMG (257) lists the different types of poisons that can be found. They are:
- Contact poisons
- Ingested poisons
- Inhaled poisons
- Injury poisons
Effects of Poison
Every poison has its own effect which I will summarize here. (I did not list any poisons only available to some characters such as the ones found in class features and magic items)
Listed with a price
These are the poisons that seem to be more available to players:
- Assassin's Blood - Constitution save (damage and poisoned cond.) [DMG 257]
- Basic Poison - Constitution save (damage) [PHB 153]
- Burnt Othur Fumes - Constitution save (damage over time and poisoned cond.) [DMG 258]
- Carrion Crawler Mucus - Constitution save (poisoned and paralyzed cond.) [DMG 258]
- Drow Poison - Constitution save (poisoned and unconscious cond.) [DMG 258]
- Essence of Ether - Constitution save (poisoned and unconscious cond.) [DMG 258]
- Malice - Constitution save (poisoned and blinded cond.) [DMG 258]
- Midnight Tears - Constitution save (damage) [DMG 258]
- Oil of Taggit - Constitution save (poisoned and unconscious cond.) [DMG 258]
- Pale Tincture - Constitution save (damage over time and poisoned cond.) [DMG 258]
- Purple Worm Poison - Constitution save (damage) [DMG 258]
- Serpent Venom - Constitution save (damage) [DMG 258]
- Torpor - Constitution save (poisoned and incapacitated cond.) [DMG 258]
- Truth Serum - Constitution save (poisoned cond. and zone of truth effect) [DMG 258]
- Wyvern Poison - Constitution save (damage) [DMG 258]
Poisonous Plants
- Yellow Mold - Constitution save (poisoned cond.) [DMG 105]
Monster Poisons
Many monsters have poisons attached to their attacks. I chose not to list them here, because there are just so many. Some of these monsters have explicitly harvest-able poisons (using the rules in the DMG), but others do not. Ask your GM if you could gain access to these other poisons by harvesting them or collecting them from the creatures in question.
Most of the ones I've found are in the MM, GGtR, and a variety of adventure modules.
Ways to Use Poison
The rules for this are mostly found in the DMG (258), where rules for crafting, harvesting, and purchasing poisons can be found. XGtE (128) offers an alternative ruleset to item crafting.