Leech Seed drain HP at the end of each turn.
In battles involving multiple Pokémon, Leech Seed will only heal the Pokémon that is in the same position as the Pokémon that originally used Leech Seed was in. If no Pokémon is in that position (for example, if it fainted and could not be replaced), Leech Seed will not take any health from the seeded Pokémon. However, if a Pokémon is later able to occupy that position (if it was revived), Leech Seed will once again sap HP from the seeded Pokémon. While a Pokémon is seeded, even if it cannot have its health drained due to there being no Pokémon in the appropriate slot to drain its HP, it cannot be affected by Leech Seed again.