Leech Seed plants a seed on the target. At the end of each turn that the target is under the effect of Leech Seed, 1/16 of the target's HP will be drained, and the same amount of HP will be restored to the target's opponent (even if it was not the Pokémon that originally used the attack). Recurrent Leech Seed draining will not occur if the seeded Pokémon defeats the target. Unlike other types of damage, the recurrent damage done by Leech Seed can exceed the target's current HP. Leech Seed will not work against Grass-type Pokémon, and attempts to seed an already-seeded Pokémon will fail.
If the target has less than 16 HP, Leech Seed will drain 1 HP per turn. Leech Seed draining will occur even if the seeded Pokémon knocks itself out with crash damage, recoil damage, or self-inflicted confusion damage, or by making a substitute before the end of its turn. Draining from Leech Seed is done after continuing partial trapping damage and recurrent poison or burn damage. Leech Seed can be removed by Haze or switching. If the target is seeded during a round that it switches in, it will take recurrent damage during that round, but only if its speed is less than or equal to the user's speed.
If the target of Leech Seed is also under the effect of Toxic, Leech Seed damage will increase every turn as the N value of Toxic increases because Leech Seed and Toxic both use the same damage algorithm ( N × max (1, int (0.0625 × Max HP)). If the current HP of the recipient of Leech Seed's HP-restoring effect is greater than its maximum HP, its current HP will be set equal to its maximum HP.
1/8 of the target's HP is drained instead. Rapid Spin will release a Pokémon from the effects of Leech Seed.
Also, Leech Seed's effects on the move Toxic are no longer present.
In a Double Battle, 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. This also means that for as long as a seeded Pokémon stays in battle and Leech Seed is not removed by Rapid Spin or Haze, that Pokémon will not be able to be seeded again.
In Super Smash Bros. Melee, Leech Seed is one of the random outcomes of Togepi's Metronome. All characters within the area of effect will receive a flower on their head that causes 5% damage every few seconds.
In the anime
Being unaffected by the drain
A seed is planted on the foe. It steals some HP from the foe.
A seed sprouts from Bulbasaur's bulb, which shoots out to the opponent. When hit, the opponent becomes wrapped in thin vines and leaves that restrain it. The vines and leaves then glow red and suck energy from the opponent. Sometimes, the vines have no leaves at all.
Skiploom shoots one or two seeds from the middle of the flower on top of its head towards the opponent. When hit, the opponent is wrapped in vines that restrain it. The vines then glow red and suck energy from the opponent.
Shroomish shoots multiple seeds from the hole on its head towards the opponent. When the opponent is hit, the seeds sprout vines which wrap around the opponent. The vines glow red and suck energy from the opponent.
Pumpkaboo shoots several seeds from its mouth into the ground. The seeds immediately grow into vines that latch onto a target and drain them of their energy. When a Grass-type Pokémon is seeded, the target is still bound by the roots, but they are a dull gray-brown color, and it is unaffected by the energy draining.
Jumpluff holds its arms above its head and releases a stream of cotton from the cotton puffs on its head and arms at the opponent. The cotton sticks to the opponent and sprout vines that ties up the opponent.
Sceptile touches the opponent with the tip of its tail, releasing a seed from it and planting it onto the opponent. The seed then sprouts into vines that wrap around the opponent's body and absorbs its energy, or a vine with leaves on it sprouts from one of the seeds on Sceptile's back and wraps themselves around the opponent, draining its energy.