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SPD-Down is a status ailment appearing in the Mario & Luigi series. It stands for "Speed-Down".
Mario & Luigi: Superstar Saga / Mario & Luigi: Superstar Saga + Bowser's Minions
In Mario & Luigi: Superstar Saga, some enemies have the ability to change the Mario Bros' stats. If their speed goes down, enemies may get to attack before Mario or Luigi. The ailment is indicated by a streaking blue flash with an arrow pointing down.
Mario & Luigi: Partners in Time
SPD-Down is the same status ailment in Mario & Luigi: Partners in Time. It is indicated by a shrinking blue circle around the afflicted. Ice Flowers can randomly inflict this status ailment on enemies.
Mario & Luigi: Bowser's Inside Story
Mario & Luigi: Paper Jam