Synthehol is a synthetic alcohol substitute that mimics the taste and pleasant effects of alcohol, which drinkers can dismiss at will without a hangover.
Synthehol was introduced to the Federation by the Ferengi and became the standard drink aboard 24th-century starships. Its effects can be shaken off immediately, letting crew members enjoy a drink without risking their judgment on duty.
Not everyone is impressed. Montgomery Scott, revived aboard the Enterprise-D in 2369, could tell at once that he was not drinking real Scotch, and Guinan kept genuine alcohol behind the bar of Ten Forward for those who wanted it.