Does wither damage use -1/-1 counters or regular damage to reduce toughness?

Wither is a keyword ability that changes the way a source deals damage to creatures. Instead of marking the creature with regular damage (which is removed at end of turn), a source with wither causes that damage to be dealt as -1/-1 counters placed on the creature (CR 702.79a).

This is a crucial distinction: regular damage on a creature is removed during the cleanup step at the end of each turn (CR 514.2), meaning it only temporarily reduces effective toughness. -1/-1 counters, however, are permanent and remain on the creature indefinitely until removed by another effect (CR 122.1).

The number of -1/-1 counters placed equals the amount of damage the wither source would deal. So if a 2/2 creature with wither deals 1 damage to a 3/3 creature, that 3/3 becomes a 2/2 permanently, even after the turn ends. If a creature ever has 0 or less toughness due to these counters, it is put into the graveyard as a state-based action (CR 704.5f).

Example: Your opponent controls a 3/3 creature. You attack with Scourge of the Nobilis-style 2/2 creature with wither. It deals 2 damage — instead of 2 damage markers, two -1/-1 counters are placed on the opponent's 3/3, making it a 1/1 permanently even after the turn ends.

Note: Wither interacts with infect (CR 702.90a) in that both use -1/-1 counters on creatures, but infect also uses poison counters on players, while wither deals normal damage to players.