What it does
Acid Spray is a Poison-type special move with a guaranteed Special Defense drop of two stages on hit. Two stages means the target takes ×2 special damage from all subsequent hits — equivalent to halving their Special Defense for the rest of the time they're in.
In doubles, this plays differently than in singles. Both your active Pokémon can immediately benefit from the Special Defense drop. The key is turn order: if your Acid Spray user moves first, then your partner's special attack lands on the same turn and already hits into the weakened Special Defense. If your partner moves before the Acid Spray user, the drop benefits a future turn instead.
The base power is modest — Acid Spray is a support tool, not a damage source. Don't use it expecting meaningful damage output.
Who runs it
Glimmora occasionally carries it as part of a special offense setup, though its usage rate is low.
Arbok (pure Poison) uses it with STAB for team special attack support.
Salazzle (Poison/Fire) uses it to amplify its own Fire-type special attacks or those of a partner.
Flapple (Grass/Dragon) occasionally uses it as a support tool when it's built to enable a partner special attacker rather than to deal damage itself.
How to use it
The optimal play in doubles: your Acid Spray user moves first in the turn, drops the target's Special Defense by two stages, then your partner's special move hits into the weakened Defense on the same turn.
That means the Acid Spray user needs higher Speed than the partner special attacker. When that condition is met, a single turn gives you a doubled special hit on that target.
Steel-type Pokémon are immune — Acid Spray does nothing against them. Poison-type Pokémon resist the move (×0.5 damage) but are not immune; they take reduced damage and the Special Defense drop still applies. The Special Defense drop is the whole point; the damage is incidental.
Tips & strategy
Acid Spray is a narrow tool. In practice, it mainly appears in team archetypes built around enabling a high-Special-Attack partner. If your team has a Pokémon with extreme Special Attack but limited setup options, this provides a reliable one-turn amplification.
Competitors to know: Icy Wind hits both opponents and drops Speed, Screech drops physical Defense two stages, Snarl drops special attack. These are all support moves in the same general category. Acid Spray's distinction is the two-stage Special Defense drop specifically — and the Poison typing that lets it hit Fairy types super-effectively. Choosing it means your team is explicitly built around special damage amplification.