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Deals damage. The user recovers half of the HP lost by the target, rounded up. If Big Root is held by the user, the HP recovered is 1.3x normal, rounded half down. If the target has the ability Liquid Ooze, no HP will be restored and the user will take damage equal to the amount it would otherwise have healed instead.
Mega Drain should not be used competitively, as it is completely outclassed by Giga Drain.
Giga Drain is a more powerful version of this move.
Energy Ball is a more powerful alternative that has a chance to lower the opponent's Special Defense stat by one stage, but it does not heal the user.
Leaf Storm is a much more powerful alternative, but it lowers the user's Special Attack stat by two stages when it is used and does not heal the user.
Horn Leech is a similar, more powerful physical Grass-type move.