Nice Guy Potion

Nice Guy Potion is a concoction created by Kooky Von Koopa, which forces anyone who inhales it to act and behave like a kind, sensitive person, even if the being it's used on is naturaly mean and spiteful.

Nice Guy Potion was used only in The Adventures of Super Mario Bros. 3 episode Princess Toadstool for President. After overhearing how his father couldn't act nicely enough in order to win the votes of the Mushroom People in an election against Princess Toadstool, Kooky sprayed King Koopa with a dose of Nice Guy Potion, which made him act like a nice guy for a undetermined amount of time.

The potion eventually wore off, and King Koopa returned to being his selfish old self. Although the situation could've been remedied with another dose of Nice Guy Potion, Mario disposed the the second batch prepared by Kooky and Cheatsy. With no potion forcing him to be nice, King Koopa blew his chance at winning the election by accidentally scaring the Mushroom People. As a result, the only vote he received was his own.

Trivia
The Nice Guy Potion is blue when Kooky sprays it in King Koopa's face. But when Kooky and Cheatsy prepare the potion in the lab, it's purple. It is possible that the final potion is blue, and that the purple potion was unfinished. However, this might just be an animation goof.