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Slow is a status ailment found in Paper Mario: The Thousand-Year Door. Beldam is the only one who can give this status ailment to Mario or his partner. Mario or his partner can give the Slow status, but it cannot be done normally as the item, known as the Turtle Curse, required to do so must be hacked in the game. When this effect is used, it will say "Less chances to attack are now available".
However, there is a way to do this indirectly: Mario or his partner must confuse an enemy with something like Ruin Powder or Head Rattle, and if the enemy uses a Slow-inducing attack on an adjacent enemy, like a confused Beldam causing slow on Marilyn.
When afflicted, Mario or his partner can only attack once every other turn. It is indicated by a sad purple face. The status ailment reoccurs in the sequel, Super Paper Mario. In this case, however, it is inflicted by the common Cursya and instead in this game it just says the word "Slow". One of Count Bleck's moves is like the same as the slow status.
If a target has the Slow status effect and is inflicted with Fast, the two effects cancel each other out.