The decorpsinator serum is a concoction that is capable of reviving dead tissue. It was created by Princess Bubblegum in the episode "Slumber Party Panic." When she initially tested it on dead Candy People, it was incomplete and had a green color. It created an army of undead zombies that threatened to destroy the Candy Kingdom. Later on, after Finn solved a simple math question, Princess Bubblegum took the answer, four, and applied it to her decorpsinator serum, which finally made it complete and turned it orange. She then used it to resurrect the undead Candy People that had tried to attack earlier in the episode. This time, the serum brought them back to the condition they were in while still alive.
The only ingredient mentioned in "Slumber Party Panic" is explosive diarrhoea. In "From Bad to Worse", one of the liquids which Science tells Finn to add to the potion is called knife juice.
Several other ingredients are named in unused storyboards from "Slumber Party Panic", including baking soda, CO2 sauce, hydrogen sprinkles, selenium, nutmeg, red #5, and a can of magnapple.