What happens if I have Miltank with the "Moomoo Malt" Ability and my opponent has Gengar with the "Gnawing Curse" Ability in play? Which Ability goes first? Or does it even matter?

The player that owns the affected Pokemon gets to choose the order that the two simultaneous Abilities are applied. However, keep in mind that you cannot go below zero damage done; for example, if you have 50 damage and you use Moomoo Malt first, you can't "go negative" and then have Gnawing Curse's counters simply disappear. Also remember, if you choose Gnawing Curse first and the damage counters put the Pokemon over its maximum HP you still get to use Moomoo Malt before checking for KO's.

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