Leer will lower the opponent's Defense by 1 step (termed BTLs in the Gameboy description) on the Stat growth scale.
Leer will always fail if it has already been used 6 times on the same pokemon, without a move that raises Defense being used by or against the pokemon affected by Leer. This is due to the fact that a stat may only be lowered by a maximum of 6 steps, until it is reset by the use of Haze or the pokemon is switched out normally. The maximum is the limit of how many decreases can be applied to a stat. If another move decreases the Defense stat of the pokemon that Leer is used against, the number of steps that it decreases the Defense stat by will count toward the 6 step limit, and reduce the number of times that Leer can be used appropriately.
Leer is affected by Brightpowder when the opposing pokémon is equipped with it.
Like all other non-damaging Normal-type attacks, Leer affects Ghost-type pokémon, despite their immunity to Normal attacks.
