Do -1/-1 counters and +1/+1 counters on the same creature cancel each other out immediately?

Whenever a creature has both +1/+1 counters and -1/-1 counters on it at the same time, the game automatically removes pairs of one of each until only one type remains (or neither). This happens via state-based actions, which the game checks continuously whenever a player would receive priority.

Specifically, CR 704.5q states: 'If a permanent has both a +1/+1 counter and a -1/-1 counter on it, N +1/+1 and N -1/-1 counters are removed from it, where N is the smaller of the number of +1/+1 and -1/-1 counters on it.' This means they annihilate in equal pairs — neither counter type 'wins'; they simply cancel out one-for-one.

This is important because it means you can never have both types present simultaneously in a stable game state. The moment both types exist on a creature, the state-based action fires and removes them before any player can act.

Example: Your 3/3 creature has two +1/+1 counters on it (making it a 5/5 effectively). An opponent uses Grief's effect to put three -1/-1 counters on it. State-based actions immediately remove two pairs, leaving only one -1/-1 counter. The creature is now a 2/2 (3/3 base, -1/-1 net).

Note: the annihilation only removes counters — it does not deal damage or cause any triggered abilities tied to counters being added. The net effect on power and toughness is the same as if you had just done the math directly.