When a Boo tries to steal coins or a Star from a player holding a Boo Repellant, the item automatically activates, spraying gas at the Boo, causing it to flee. Boo Repellant can only be used once. It is also possible for a player to throw away a Boo Repellant at the start of their turn to make room in their inventory, although it would count as using an item during the user's turn.
The AI is not programmed to check if a player has a Boo Repellant before attempting to steal from them; they will even attempt to steal Stars from players who have one.
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