Fusel alcohols, also sometimes called fusel oils, are higher order (more than two carbons) alcohols formed by fermentation and present in cider, mead, beer, wine, and spirits to varying degrees.

The compounds involved are chiefly:

• 1-propanol
• 2-propanol
• butanol (various isomers)
• amyl alcohol
• furfural

Excessive concentrations of these fractions can cause off flavours, sometimes described as "spicy," "hot," or "solvent-like." Some beverages, such as whiskey, Siwucha and traditional ales and ciders, are expected to have relatively high concentrations of fusel alcohols as part of the flavor profile. In other beverages, such as vodka and lagers, notable presence of fusel alcohols is considered a fault. Very high concentrations — usually caused by incompetent distillation — can cause acute illness, including headaches, nausea, vomiting, clinical depression, or coma. Such a liquor may be referred to as rot-gut or rotgut.

Fusel alcohols are formed when fermentation occurs:

• at higher temperatures,
• at lower pH,
• when yeast activity is limited by low nitrogen content.

During distillation, fusel alcohols are concentrated in the "tails" at the end of the distillation run. They have an oily consistency, which is noticeable to the distiller, hence the other name fusel oil. If desired, these heavier alcohols can be almost completely separated in a reflux still.