Fusel alcohols, also sometimes called fusel oils, or potato oil in Europe, are a mixture of several alcohols (chiefly amyl alcohol) produced as a by-product of alcoholic fermentation. The word Fusel is German for "bad liquor".
Hazardous alcohols 
- Methanol (methyl alcohol) itself, while poisonous (LD50 5628 mg/kg, oral, rat), has a much weaker sedative effect than ethanol.
- Isopropanol (isopropyl alcohol) is oxidized to form acetone by alcohol dehydrogenase in the liver but have occasionally been abused by alcoholics, leading to a range of adverse health effects.
Alcohol faults 
- Mannitol, a wine fault
Aroma alcohols 
Excessive concentrations of some alcohols other than ethanol may cause off-flavors, sometimes described as "spicy", "hot", or "solvent-like". Some beverages, such as rum, whisky (especially Bourbon), incompletely rectified vodka (e.g. Siwucha), and traditional ales and ciders, are expected to have relatively high concentrations of non-hazardous alcohols as part of their flavor profile. However, in other beverages, such as Korn, vodka, and lagers, the presence of other alcohols than ethanol is considered a fault.
The compounds involved are chiefly:
- 2-methyl-1-butanol - sometimes called "active" amyl alcohol
- isoamyl alcohol - also known as isopentyl alcohol
- isobutyl alcohol - one of the least toxic of the butanols.
- n-propyl alcohol
Formation and removal 
- at higher temperatures
- at lower pH
- when yeast activity is limited by low nitrogen content
During distillation, fusel alcohols are concentrated in the feints or "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. Freeze distillation, on the other hand, does not remove fusel alcohols.
Health effects 
There is a popular belief that fusel alcohol contributes to hangover symptoms. However, these effects are under scientific debate. A Japanese study in 2003 concluded, "The fusel oil in whiskey had no effect on the ethanol-induced emetic response" in Suncus murinus. Additionally, consumption of fusel oils with ethanol suppressed subjects' subsequent taste aversion to alcohol, which suggested subjects' hangover symptoms were lessened.
See also 
- Hazelwood, Lucie A.; Daran, Jean-Marc; van Maris, Antonius J. A.; Pronk, Jack T.; Dickinson, J. Richard (2008). "The Ehrlich pathway for fusel alcohol production: a century of research on Saccharomyces cerevisiae metabolism". Appl. Environ. Microbiol. 74 (8): 2259–66. doi:10.1128/AEM.02625-07. PMC 2293160. PMID 18281432..
- Wiernikowski, A, Piekoszewski, W, Krzyzanowska-Kierepka, E, Gomułka, E (1997). "Acute oral poisoning with isopropyl alcohol in alcoholics". Przeglad lekarski 54 (6): 459–63. PMID 9333902.
- Mańkowski, W, Klimaszyk, D, Krupiński, B (2000). "How to differentiate acute isopropanol poisoning from ethanol intoxication? -- a case report". Przeglad lekarski 57 (10): 588–90. PMID 11199895.
- Aroma of Beer, Wine and Distilled Alcoholic Beverages
- MERCK INDEX 10th Ed. 1983
- Hori H, Fujii W, Hatanaka Y, Suwa Y. (Aug 27, 2003). "Effects of fusel oil on animal hangover models.". Alcohol Clin Exp Res. 27 (8 Suppl): 37S–41S. doi:10.1097/01.ALC.0000078828.49740.48. PMID 12960505.