Alcohol tolerance refers to a decreased response to the effects of ethanol in alcoholic beverages. This reduced sensitivity requires that higher quantities of alcohol be consumed in order to achieve the same effects as before tolerance began to occur. Ethanol, also known as ethyl alcohol, drinking alcohol or grain alcohol, is a flammable, colorless, slightly toxic chemical compound with a distinctive perfume-like odor, and is the alcohol found in alcoholic beverages. ... Drunkenness, in its most common usage, is the state of being intoxicated by consumption of ethyl alcohol to a degree that mental and physical facilities are noticeably impaired. ...

Laws establishing maximum blood alcohol concentration (BAC) levels for legal operation of motor vehicles do not take into consideration alcohol tolerance. In one study, a group of alcoholics whose BAC was brought to .10, only 24% showed any clinical signs of impairment. Blood Alcohol Content (or Blood Alcohol Concentration), often abbreviated BAC, is the concentration of alcohol in blood, measured, by volume, as a percentage. ...

The alcohol tolerance is connected with activity of Alcohol dehydrogenases (a group of enzymes responsible for the breakdown of alcohol) in the liver, and in the bloodstream. High level of Alcohol dehydrogenase activity results in fast transformation of ethanol to more toxic acetaldehyde. Such atypical alcohol dehydrogenase is less frequent in alcoholics than in nonalcoholics. Furthermore, among alcoholics, the carriers of this atypical enzyme consume lower ethanol doses, compared to the individuals without the allele. Alcohol Dehydrogenase Alcohol dehydrogenases are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones. ... Ribbon diagram of the enzyme TIM, surrounded by the space-filling model of the protein. ...

Some persons of South-East Asian and near-Eastern descent have a mutation in their alcohol dehydrogenase gene, making this enzyme more active.

Heavy alcohol consumption over a period of years can lead to "reverse tolerance". A liver can be damaged by alcohol abuse leading to a buildup of fat and scar tissue. The reduced ability of such a liver to metabolize or break down alcohol means that small amounts can lead to a high BAC and resulting intoxication. Santorio Santorio (1561-1636) in his steelyard balance, from Ars de statica medecina, first published 1614 Metabolism (from μεταβολισμος(metavallo), the Greek word for change), in the most general sense, is the ingestion and breakdown of complex compounds, coupled... ...

References

• Carroll, Charles R. Drugs in Modern Society. NY: McGraw-Hill, 2000 (fifth ed.).
• Chesher, G., & Greeley, J. Tolerance to the effects of alcohol. Alcohol, Drugs and Driving, 1992, 8(2):93-106.
• Osier M., Pakstis A.J., Kidd J.R., Lee J.F., Yin S.J., Ko H.C., Edenberg H.J., Lu R.B., Kidd K.K. Linkage disequilibrium at the ADH2 and ADH3 loci and risk of alcoholism // Am. J. Hum. Genet. 1999, 6: 1147-1157.
• Muramatsu T., Zu-Cheng W., Yi-Ru F., Kou-Bao H., Heqin Y., Yamada K., Higuchi S., Harada S., Kono H. Alcohol and aldehyde dehydrogenase genotypes and drinking behavior of Chinese living in Shanghai // Hum. Genet. 1995,96: 151-154.
• Neumark Y.D., Friedlander Y., Thomasson H.R., Li T.K. Association of the ADH2*2 allele with reduced ethanol consumption in Jewish men in Israel: a pilot study // J. Stud. Alcohol. 1998, 59: 133-139.
• Osier M.V., Pakstis A.J., Soodyall H., Comas D., Goldman D., Odunsi A., Okonofua F., Parnas J., Schulz L.O., Bertranpetit J., Bonne-Tamir B., Lu R.B., Kidd J.R., Kidd K.K. A global perspective on genetic variation at the ADH genes reveals unusual patterns of linkage disequilibrium and diversity // Am. J. Hum. Genet. 2002,71: 84-99.
• Borinskaya S. A., Gasemianrodsari F., Kalyina N. R., Sokolova M. V., Yankovsky N. K. Polymorphism of alcohol dehydrogenase gene ADH1B in eastern Slavic and Iranian-speaking populations. Genetika. 2005, 41: 1563-1566 (in Russian).