Synthehol was a synthetic variant of alcohol that had no intoxicating and addictive properties.
Synthehol was invented by the Ferengi. Ferengi merchants used it in social settings in which they would serve drinks with actual alcohol to customers while they consumed synthehol. Soon the hapless victim would be totally drunk and making all sorts of deals with a Ferengi merchant who was still quite sober. Eventually the secret of synthehol was made public. At first irritated over the loss of their secret, the Ferengi got over their irritation when straight sales of synthehol turned out to be quite sizable.
Synthehol subsequently became a popular beverage in the Federation. Despite not having any of the deleterious effects of alcohol, synthehol was still considered an adult beverage. (TNG novel: Strike Zone) Some individuals had negative attitudes towards synthehol and believe it affected the taste of drinks. (TNG episodes: "Up the Long Ladder", "Family", "Relics"; DS9 episode: "Body Parts")
In 2369, during Quark's brief time as Grand Nagus of the Ferengi Alliance, Nava approached him and asked permission to begin selling synthehol in the Gamma Quadrant. Nava agreed to provide Quark with fifty percent of the synthehol profits. (DS9 episode: "The Nagus")