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Deals damage and has a 100% chance to lower the target's Special Defense by two stages.
While Acid Spray is certainly not the most powerful of moves, it has an extremely good special effect, namely that it will effectively halve the Special Defense stat of its target after its first use, making the opponent far more vulnerable to the user's special attacks as a result. However, it is limited in its application by relatively low distribution, and most of the Pokemon that do get the move cannot use it effectively on account of low Special Attack or general offensive incompetence. Some exceptions here are Tentacruel and Eelektross, both of whom have the bulk and the power to use the move effectively.
Sludge Bomb is a much stronger alternative that doesn't lower the opponent's Special Defense and has a chance to poison the opponent.