Wither is a harmful (negative) status effect in Minecraft Java Edition 1.21.
minecraft:wither
Give Wither II for 60 seconds (Java Edition 1.13+; duration in seconds, then amplifier where 0 = level I):
Remove the effect from the nearest player:
Wither drains health steadily and turns your hearts black, and unlike poison it can kill you outright, making it one of the deadliest effects in the game.
The Wither boss inflicts it with its skull projectiles, and wither roses apply it on contact to anything walking through. Suspicious stew crafted with a wither rose carries the effect, which is a real trap for the unwary.
Wither bypasses the comfort of poison's half-heart floor, so treat black hearts as an emergency: drink milk, eat, and back off. Wither roses from Wither-killed mobs are worth farming for mob farms, since the effect finishes off damaged mobs automatically.
The effect ID for Wither is minecraft:wither. Java Edition 1.13+ /effect commands use this namespaced form. On Java 1.12.2 and earlier the numerical ID was 20.
Run /effect give @p minecraft:wither 60 1 to get Wither 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:wither.
Wither is a harmful (negative) status effect in Minecraft Java Edition, based on its registry category (harmful).
Both drain health over time, but Poison stops at half a heart while Wither keeps going and kills. Wither also blackens your health bar so you cannot see your exact hearts.
Drink milk to remove it instantly, or out-heal it with regeneration and food until it expires. Avoid the source: Wither skulls and wither roses reapply it on contact.