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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.
Giga Drain got a power boost in BW and now has a usable 75 Base Power; for that reason it became more well known on several Grass-types that enjoy the massive recovery that it can now provide. From defensive Pokemon to offensive sets, this move became a staple thanks to the additional survivability it gives. A honorable mention goes to Celebi; thanks to its great natural bulk and expansive movepool, the forest pixie makes good use of this move regardless of whether it's running an offensive or defensive set.
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 users Special Attack stat by two stages every time used and does not heal the user.
Horn Leech is a physical Grass-type equivalent.