Twineedle deals damage and will strike twice (with 25 base power each time). It has a 20% chance of poisoning the target. Poison or Steel Pokémon, those with the ability Immunity or those behind a Substitute cannot be poisoned. Each strike of Twineedle is treated like a separate attack: Each strike can get a critical hit independently. Abilities that activate on contact (Static, Weak Armor, Iron Barbs etc.) activate for each strike. If the Sturdy ability, Focus Sash, or Focus Band activates before the final hit, the subsequent hit will still deal damage and therefore cause the opponent to faint. If the first hit breaks a Substitute, the second hit will deal damage to the target Pokémon. The move Counter only counters the final hit, but Bide counters the complete damage.
In Generations 1-2, Twineedle can poison Poison and Steels. In Generations 2-4, if a Focus Sash or Focus Band activates before the final hit, subsequent hits do not cause the holder to faint. In Generations 1-3, Twineedle ends immediately if any hit breaks a Substitute. In Generation 1, each hit will always deal the same damage – if the first critical hits, so will the rest.