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Deals damage and has a 10% chance to lower the target's Special Defense by one stage.
Acid is, like most low-power moves, completely useless competitively. While Acid Spray is at least moderately useful thanks to its excellent secondary effect, Acid has no such power, so is unfortunately doomed to obscurity. If you want to lower the opponent's Special Defense, Acid Spray is easily better. For a more powerful special Poison-type move, Sludge Wave or Sludge Bomb are both more than twice as powerful and have a much wider circulation, thanks to their status as TMs. Don't bother using it, ever.
Acid Spray has the same power but has a 100% chance to lower the opposing Pokemon's Special Defense by two stages.
Sludge Bomb is a much more powerful alternative with a chance to poison the opponent.