Prune juice
Prune juice is a sweet Terran beverage made from the juice of the prune, or dried plum fruit, of Earth.
Although generally unpopular among Humans, prune juice was noted by Worf as being "a warrior's drink", when he was introduced to it by Guinan aboard the USS Enterprise-D in 2366. (TNG: "Yesterday's Enterprise")
Sergey and Helena Rozhenko were surprised when Guinan told them Worf enjoyed the drink, as he had never shown an interest in human foods and beverages with them. (TNG: "Family")
Worf often ordered chilled prune juice at Quark's on Deep Space 9, as well as in the mess hall aboard the USS Defiant. (DS9: "The Way of the Warrior", "The Quickening")
Worf often ordered an extra large juice. (DS9: "To the Death", "Let He Who Is Without Sin...")
Quark recounted an occasion that Worf toasted Ch'Pok with prune juice at Worf's hearing in 2372. (DS9: "Rules of Engagement")
Jadzia Dax once told Worf that "for a Klingon who was raised by humans, wears a Starfleet uniform, and drinks prune juice, you're pretty attached to 'tradition'."
While pining for Grilka, Worf took solace in the drink. (DS9: "Looking for par'Mach in All the Wrong Places")
Doctor Bashir prescribed to Odo, who was at the time living in humanoid form, that he had to relax. Odo later admitted: "And it helped... that and the prune juice." This conversation could indicate that Odo was suffering from a mild constipation at the time. (DS9: "The Begotten")