Romulan ale is one of the most potent alcoholic beverages in the Star Trek universe, but this wasn't why it was made illegal by the United Federation of Planets. Despite Romulan ale being contraband, it still found its way into the hands of many Starfleet officers over the years. As a result, several Star Trek characters have nursed Romulan ale-induced hangovers, from Captain James T Kirk (William Shatner) to Lieutenant Commander Worf (Michael Dorn), but the reasons for its illegality were more political.
From the period immediately after the Earth-Romulan War to the alliance during the Dominion War, relations between the Federation and the Romulan Star Empire were fraught. As part of the Treaty of Algeron that brought an end to the war, Federation scientists were forbidden from developing cloaking technology, and the Romulan Neutral Zone was established to mark a barrier between Federation-occupied space and the Romulan Star Empire. These terms played a big part in the outlawing of Romulan ale in the Star Trek universe, and here's the history of the intoxicating beverage, and the changes in its legal status.
The History Of Romulan Ale In Star Trek 
Although it's not named, Romulan ale first appears in the Star Trek: The Original Series episode, "The Enterprise Incident". When Lieutenant Commander Spock (Leonard Nimoy) infiltrated a Romulan battle cruiser, he drinks a blue drink with a female Romulan commander (Joanne Linville). Later, in Star Trek II: The Wrath of Khan, Doctor Leonard McCoy visited Kirk to commiserate the latter's 52nd birthday, bringing an illegal bottle of Romulan ale for the two to enjoy. Years later, Kirk had Romulan ale served at a diplomatic function with visiting Klingon dignitaries, although he wasn't prepared for his hangover to be exacerbated when his anti-Klingon prejudice saw him accused of assassinating Chancellor Gorkon (David Warner).
