The instant bath serum was made by Princess Bubblegum. The serum eliminates the user's need to shower by making the drinker extremely feverish, having the drinker sweat cleaning agents. Because Finn doesn't bathe, he was prompted to test it for Princess Bubblegum in "Too Young." It turned out to be incredibly spicy. Later in the episode, Princess Bubblegum made a second batch that was even spicier than the previous bottle that Finn used to help him prank the Earl of Lemongrab by trying to spice all of the Earl's food.
Peppermint Butler after a drop of Instant Bath Serum is accidentally dropped in his eye
Lemongrab after tasting carrots with the Instant Bath Serum in it