Toxic causes the target to be
badly poisoned. Pokémon that are badly poisoned will lose an increasing amount of HP at the end of each turn.
The affected Pokémon will lose 1⁄16 of its maximum HP at the end of the first turn, which then increases by 1⁄16 each turn. In other words, they lose 1⁄16, 2⁄16, 3⁄16, 4⁄16 and so on.
Toxic doesn't affect Pokémon already carrying a major status ailment, Pokémon behind a Substitute nor Poison- or Steel-type Pokémon.