Synthehol is an ethanol substitute widely served in adult drinks in the Federation. Reportedly, it does not have the "deleterious effects" of ordinary alcohol, and its intoxicating effects can be "easily dismissed" if the drinker needs to function at normal capacity due to an emergency.
Synthehol's effects seem to be "easily dismissed" because humans and most humanoids metabolize it much more rapidly than alcohol. Having lived for years as a Borg drone, Seven of Nine proved unable to metabolize synthehol efficiently, so it affected her much like ordinary alcohol. Some alien races are also unable to metabolize synthehol and are similarly affected as if it were normal alcohol.
Many people can apparently distinguish synthehol from alcohol by taste and find synthehol unpleasant.