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Steals 1/16 of the target's HP after it attacks and gives it to the user. This attack's effect will last until the opposing Pokémon either switches out or faints. It has no effect on Grass Pokémon or Substitutes. Leech Seed can be combined with Toxic to steal more HP each turn. The effects of Leech Seed are negated if the move Haze is used.
Leech Seed is not as good in RBY as it is in later generations. Even when used with Toxic, which doubles every turn Leech Seed inflicts damage, it's not a good option, as switching out will simply cancel Leech Seed and turn Toxic into normal poison. All in all, a poor option for Grass-types.