Dum Dum Juice (also known as "Happy Juice") is a potion created by Princess Bubblegum's Uncle Gumbald. It affects Candy People, causing them to become more docile, simple-minded, child-like, and stupid.
As a child, Bonnibel Bubblegum was very lonely, living in a nearly uninhabited wasteland. She decided to create herself family members. Doing so she created Aunt Lolly, Cousin Chicle, and Uncle Gumbald. She and the family began to build a candy town, but doing so Gumbald came up with the idea of building a candy city. Bonnie however did not like this idea and used a butterscotch bomb to create a lake where Gumbald intended to build the city. This infuriated Gumbald but came up with a plan for revenge. Gumbald created his niece a "boyfriend", Mr. Cream Puff, who was rather stupid and simple-minded. Gumbald and the family sisterly sent Bonnie away with Mr. Cream. Bonnibel returned later that night, only to find her family talking about overthrowing her. Gumbald created the Dum Dum Juice to make her more docile and simple-minded, as her boyfriend, Mr. Cream Puff, is. Gumbald however betrayed Lolly and Chicle, giving them cupcakes filled with the juice. Eating the cupcakes, Chicle became the brown candy ball known as Crunchy, while Lolly became known as Manfried the talking piñata, both forgetting their previous identies. Bonnibel was furious when catching this and became cornered by her uncle. Gumbald intended to splash Bonnie with the juice, but she was holding a peashooter, which she used to break the glass beaker holding the juice. Thusly, Gumbald was affected by the Dum Dum Juice, forgetting his former self and became the Punch Bowl. For the next 800 years, the three remained in their new candy forms as Bonnie believed they were happier that way, with most people not knowing their true origin.
It is likely that Princess Bubblegum used the Dum Dum Juice while creating a lot of the Candy People, as most are docile and stupid. However Peppermint Butler, Cinnamon Bun (after becoming "fully baked" in "The Red Throne"), the Candy Tavern People, the Pup Gang do not seem to qualify as "docile and simple-minded", implying that not all candy people have been directly affected with Dum Dum Juice.
During the events of "Skyhooks II", after Lumpy Space Princess' anti-elemental "lumps" saved Ooo possessed under the elemental spell and turning everything into its "true form", the Dum Dum Juice was cancelled out thus turning the Punch Bowl, Manfried, and Crunchy, back into Gumbald, Lolly, and Chicle. Rumors about them spread around, as Crunchy was "missing", and people reported seeing Gumbald himself. In "Seventeen", Gumbald, Lolly, and Chicle revealed themselves to the Princess. The brought a gun filled with the Dum Dum Juice threatened to fire it at Bonnie, but Marceline was present, scaring the family away.
In "Gumbaldia" the Candy Kingdom and Gumbald's new kingdom, Gumbaldia were going to war. Finn, not liking this idea, went to confront Gumbald, trying to call off the war. After saving Gumbald's life, Gumbald thanked him, along with Jake. Gumbald, Lolly, and Chicle had dinner with Finn and Jake, calling off the war. Chicle brought out a bucket filled with "celebration juice" (which unbeknownst to Finn and Jake, was the Dum Dum Juice) and splashed it on Finn and Jake, telling them to "give Bonnie a big hug". He was also splashed with the Dum-Dum Juice and transformed back to Crunchy. Princess Bubblegum was thrilled to have a peace treaty, but noticed Finn and Jake's smell. Peppermint Butler went to wipe to off, but transformed into a stupid, baby-like form because of the Dum Dum Juice. After this, Princess Bubblegum once again declared war.