From the Super Mario Wiki, the Mario encyclopedia
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 used only 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.
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