Synthehol (a portmanteau of "synthesized" and "alcohol") is a chemical variant of alcohol. It appears to have the same taste and smell as "real" alcohol to most individuals, but none of the deleterious effects associated with alcohol for most humanoids, such as debilitating intoxication, addiction, and alcohol poisoning. Most humanoids have an enzyme which breaks down the alcohol-like compounds in synthehol. According to Data, synthehol's "intoxicating effects can be easily dismissed".
The slight chemical difference between synthehol and real alcohol is an annoyance to lovers of alcoholic beverages such as Robert Picard and Montgomery Scott. Robert Picard believed that synthehol corrupted his brother Jean-Luc Picard's taste for wine, while Jean-Luc believed it gave one a greater appreciation for the real thing. Captain Scott was able to differentiate alcohol from synthehol when he drank a glass of scotch in the USS Enterprise, NCC-1701-D--D 's Ten Forward lounge. He found the taste revolting, and he let the young bartender who served him know it.
On at least two occasions, Seven of Nine has become intoxicated after drinking synthehol, as her Borg implants were not designed to process it. She suffered similar symptoms to those that alcohol has on Humans.
In 2375, when the Kadi ambassador Tomin visited USS Voyager, he attended a reception at which he drank some syntheholic beverages. It was later realized that Kadi physiology lacks the enzyme which breaks down the alcohol-like substance in synthehol, which caused him to become drunk and attempt to woo Seven of Nine. He later passed out, and The Doctor was able to remove the synthehol with Seven's nanoprobes.
On some replicators, a synthehol button was available for easy manual input.