If the Defending Pokémon doesn't already have a Sludge marker on it, put a Sludge marker on it. If a player's Pokémon has a Sludge marker on it, he or she pays more to retreat that Pokémon. (Sludge markers stay on the Pokémon as long as it's in play.)
Flip a coin. If heads, the Defending Pokémon is now Poisoned.