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The user recovers HP equal to half of the damage dealt to the opponent, rounded down.
Mega Drain is a good coverage option for a lot of Pokemon in OU such as Gengar. It's good since it provides some recovery and deals with threats such as Rhydon, Golem, and even Lapras and Starmie. It should also be used by Grass-types should they lack access to Razor Leaf.
Razor Leaf is a more powerful move with a high critical hit rate.