Poison is a harmful (negative) status effect in Minecraft Java Edition 1.21.
minecraft:poison
Give Poison II for 60 seconds (Java Edition 1.13+; duration in seconds, then amplifier where 0 = level I):
Remove the effect from the nearest player:
Poison drains health steadily down to half a heart but never kills you, leaving you one hit from death in the process.
Cave spider bites, witch splash potions, pufferfish, and poisonous potatoes inflict it, and you can brew your own Potion of Poison from an awkward potion and a spider eye. Suspicious stew with lily of the valley carries it too.
Poison ticks down to a single half-heart and stops, so it is a setup tool, not a finisher: poison the target, then land one hit. Splash Poison ignores armor on most mobs, and witches drink their own antidote, so do not bother poisoning them.
The effect ID for Poison is minecraft:poison. Java Edition 1.13+ /effect commands use this namespaced form. On Java 1.12.2 and earlier the numerical ID was 19.
Run /effect give @p minecraft:poison 60 1 to get Poison II for 60 seconds. The first number is the duration in seconds; the second is the amplifier (0 = level I, 1 = level II). Remove it with /effect clear @p minecraft:poison.
Poison is a harmful (negative) status effect in Minecraft Java Edition, based on its registry category (harmful).
No. Poison stops at half a heart and cannot deal the final blow, though it leaves you fragile enough that anything can. Wither is the similar effect that does kill.
Brew a spider eye into an awkward potion. Glowstone upgrades it to Poison II, redstone extends the duration, and gunpowder makes it throwable for cave-spider-style revenge.