The secret ingredient in the Flaming Moe cocktail, as depicted in the episode "Flaming Moe's" of The Simpsons, is cough syrup. In the episode, Homer reveals that he added a significant amount of children's cough syrup to the drink, which unintentionally creates a unique and popular beverage. The Flaming Moe cocktail becomes a sensation at Moe's Tavern and leads to a conflict between Homer and Moe over the drink's success.