Ethyl alcohol, also known as ethanol or grain alcohol, is a flammable, colorless chemical compound, one of the alcohols that is most often found in alcoholic beverages. In common parlance, it is often referred to simply as alcohol. Its chemical formula is C2H5OH.

Ethanol has been known to humans since prehistory as the active ingredient of alcoholic beverages. Its isolation as a relatively pure compound was probably achieved first by Islamic alchemists who developed the art of distillation, such as Geber (721-815) and Al-Razi (864-930).

Ethanol for use in alcoholic beverages is produced by fermentation: it is a product of sugar metabolism in certain species of yeast in the absence of oxygen. The process of culturing yeast under conditions to produce alcohol is referred to as brewing. Yeasts can grow in the presence of up to only about 14% alcohol, but the concentration of alcohol in the final product can be increased by distillation.

For a mixture of ethanol and water, there is a maximum boiling azeotrope at 95% and 5% water. For this reason, fractional distillation of ethanol-water mixtures (of less than 95% ethanol) cannot yield ethanol purer than 95%. Therefore, 95% ethanol in water is a fairly common solvent.

To produce absolute ethanol, a small amount of benzene is added, and the mixture is again fractionally distilled. Benzene forms a trinary azeotrope with water and ethanol to remove the last of the water, and a binary azeotrope with ethanol removes most of the benzene. The resulting ethanol is water free, for processes that require it. However, several ppm of benzene remains, so consumption by humans leads to distinctive liver damage.

Ethanol is also used as a fuel and in a wide variety of industrial processes. Ethanol for industrial use is often made from petroleum feedstocks, typically by the catalytic hydration of ethylene (Sulphuric acid being the usual catalyst); this is cheaper than the production by fermentation.

Ethanol for industrial use is normally made unfit for human consumption ("denatured") by the inclusion of small amounts of substances that are either toxic (such as methanol) or unpleasant (such as denatonium benzoate), thus avoiding the applicable taxes or inventory controls. Denatured ethanol has the UN number UN 1987 and toxic denatured ethanol has UN 1986.

Ethanol is used in antifreeze products for its low melting point.

It is easily soluble in water and is itself a good solvent, used in perfumes, paints and tinctures. Alcoholic drinks have a large variety of tastes because various flavor compounds are dissolved during brewing.

A solution of 70-85% of ethanol is commonly used as a disinfectant. It kills organisms by denaturing their proteins and dissolving their lipids and is effective against most bacteria and fungi, and many viruses, but is ineffective against bacterial spores. Because of this disinfectant property, alcoholic beverages can be stored for a long time.

Alcoholic beverages have been widely used since the remote antiquity by many civilizations around the world, as a component of the standard diet, for hygienic or medical reasons, for their relaxant and euphoric effects, for recreational purposes, artistic inspiration, as aphrodisiacs, and other reasons. Some have been invested with symbolic or religious significance suggesting the mystical use of alcohol, e.g. by Greek religion in the ecstatic rituals of Dionysus, god of wine; in the Christian Eucharist; and at the Jewish Passover.

However, some people are prone to developing a chemical dependency to alcohol, alcoholism. The results of alcoholism are considered a major health problem in many nations.

Frequent excessive consumption can harmfully interfere with the user's social and financial well-being. The neurological effects of alcohol use are often a factor in deadly motor vehicle accidents and fights. People under the influence of alcohol sometimes end up in dangerous or compromising situations where they would not remain if sober.

Some religions — most notably Islam, Latter-day Saints and the Hinayanist schools of Buddhism, and some Protestant sects of Christianity forbid or discourage the consumption of alcoholic beverages for these and other reasons.

The concentration of alcohol in an alcoholic beverage may be specified in percent alcohol by volume (ABV), in percentage by weight (sometimes abbrieviated w/w for weight for weight), or in proof. The 'proof' measurement roughly corresponds in a 2:1 ratio to percent alcoholic content by volume (e.g. 80 proof = 40% ABV)

Most yeasts cannot grow when the concentration of alcohol is higher than about 18% by volume, so that is a practical limit for the strength of fermented beverages such as wine, beer, and sake. Strains of yeast have been developed that can survive in solutions of up to 25% alcohol by volume, but these were bred for ethanol fuel production, not beverage production. Liquors are produced by distillation of a fermented product, concentrating the alcohol and eliminating some of the by-products. Many wines are fortified wines with additional grain alcohol to achieve higher ABV than is easily reached using fermentation alone.

Ethanol is a moderately good solvent for many "fatty" substances and essential "oils", and thus facilitates the inclusion of several coloring, flavoring, and aromatic compounds to alcoholic beverages, especially to distilled ones. These flavoring ingredients may be naturally present in the starting material, or may be added before fermentation, before distillation, or before bottling the distilled product. Sometimes the flavor is obtained by allowing the beverage to stand for months or years in barrels made of special wood, or in bottles where scented twigs or fruits — or even insects — have been inserted.

A well-stocked bar will include a selection of beers and wines, along with the typical liqours of vodka, rum, gin, tequila, and whisky; each in varying qualities from "well" quality (off brand) to premium quality (name brand) to "top shelf" (usually very expensive, ranging from $50 to several hundred USD per 750ml bottle). Alcoholic beverages can be combined at the time of serving, sometimes with other ingredients, to create cocktails or mixed drinks. Small servings of pure liquor (shots) are also common, with whisky and tequila being traditionally popular selections.

Fermented alcoholic beverages have been known since pre-historical times. Beer was certainly known in Babylonia before 4000 BC, as attested by recipes found on clay tablets.

Wine was consumed in Classical Greece at breakfast or at symposia, and in the 1st century BC it was part of the diet of most Roman citizens. However, both Greeks and Romans generally consumed their wine watered (from 1 parts of wine to 1 part of water, to 1 part of wine to 4 parts of water). The transformation of water into wine at a wedding feast is one of the miracles attributed to Jesus in the New Testament, and his symbolic use of wine in the Last Supper led to it becoming an essential part of the Catholic Eucharist rite.

In spite of the Quranic ban on alcoholic beverages, wine (usually sold by Christian tavern-keepers) remained fairly popular in Islamic lands over the centuries, as revealed in the verses of Persian mathematician Omar Khayyam (1040–1131):

"Here with a Loaf of Bread beneath the Bough,
A Flask of Wine, a Book of Verse - and Thou
Beside me singing in the Wilderness -
And Wilderness is Paradise enow."

In Europe during the Middle Ages, beer was consumed by the whole family, thanks to a triple fermentation process — the men had the strongest, then women, then children. A document of the times mentions nuns having an allowance of six pints of ale a day. Cider and pomace wine were also widely available, while grape wine was the prerogative of the higher classes. After the collapse of the Roman Empire, wine production in Europe appears to have been sustained chiefly by monasteries.

By the times the Europeans reached the Americas in the 15th century, several native civilizations had developed alcoholic beverages. According to a post-Conquest Aztec document, consumption of the local "wine" (pulque) was generally restricted to religious ceremonies, but freely allowed to those over 70 years old (possibly the all-time record for legal drinking age). The natives of South America manufactured a beer-like product from cassava or maize (cauim, chicha), which had to be chewed before fermentation in order to turn the starch into sugars. (Curiously, the same technique was used in ancient Japan to make sake from rice and other starchy crops.)

The medicinal use of alcoholic beverages was mentioned in Sumerian and Egyptian texts dated from 2100 BC or earlier. The Hebrew Bible recommends giving alcoholic drinks to those who are dying or depressed, so that they can forget their misery.

Beer and wine are limited to a maximum 15 percent alcohol, beyond that yeast is adversely affected and can not ferment. Higher levels of alcohol have historically been obtained in a number of ways, since Babylonia in the fourth millennium BC, but it was not until the still was invented by Islamic alchemists in the 8th or 9th centuries that the history of distilled beverages begins. Alcohol appears first in Europe in the mid 12th century and by the early 14th century it spread throughout Europe. It also spread eastward, mainly by the Mongols, and was practiced in China by the 14th century. However, recent archeological evidence has supported the idea that China has had wines and distilled bevarges dating back to 5000 BC. Paracelsus gave alcohol its modern name, taking it from the Arabic word which means "finely divided", in reference to what is done to wine.

In many countries, alcoholic beverages are commonly consumed at the major daily meals (lunch and dinner).

In places and eras with poor public sanitation, such as Medieval Europe, consumption of alcoholic beverages (particularly weak or "small" beer) was one method of avoiding water-borne diseases such as the cholera. Though strong alcohol kills bacteria, the low concentration in beer or even wine will have only a limited effect. Probably the boiling of water, which is required for the brewing of beer, and the growth of yeast, which would tend to crowd out other micro-organisms, were more important than the alcohol itself. In any case, the ethanol (and possibly other ingredients) of alcoholic beverages allows them to be stored for months or years in simple wood or clay containers without spoiling, which was certainly a major factor in their popularity.

In colder climates, strong alcoholic beverages are popularly seen as a way to "warm up" the body, possibly because ethanol is a quickly absorbed source of food energy and dilates peripheral blood vessels. This however is a dangerous myth, and people experiencing hypothermia should avoid alcohol.

In many cultures, both contemporary and historical, alcoholic beverages — mostly because their neurological effects — have also played an important role in various kinds of social interaction. While other psychoactive drugs (such as opium, coca, khat, cannabis, kava-kava, etc.) also have millennial traditions of social use, only coffee and tobacco have been as universally used and accepted as ethanol is today.

Most countries have rules forbidding the sale of alcoholic beverages to children. For example, in the Netherlands and Germany, one has to be 16 to buy beer or wine and 18 to buy distilled alcoholic beverages. However, posession of alcoholic beverages is not illegal for minors in Germany. Law here is directed at the potential sellers of alcoholic beverages and not at the minors. German law puts control concerning the consumption of alcoholic beverage into the hands of custodial persons and persons with parential power.

The difference between U.S. and German law has occasionally led to severe problems with exchange pupils from the U.S. who visited Germany. The problem stems from the fact that when the pupils are in Germany they are subject to German law, but as visitors from the U.S. (and pupils of an American school) they should still obey U.S. law. Their German hosts might not have a problem serving them German beer but the pupils might be in trouble when they return to the U.S.

In law, sometimes the term "intoxicating agent" is used for a category of substances which includes alcoholic beverages and some drugs. Giving any of these substances to a person to create an abnormal condition of the mind (such as drunkenness), in order to facilitate committing a crime, may be an additional crime.

Some countries may forbid the commerce, consumption or advertising of alcoholic beverages, or restrict them in various ways. During the period known as Prohibition, from 1919 to 1933, it was illegal to manufacture, transport, import, export, or sell alcoholic beverages in the United States. Many Muslim countries, such as Saudi Arabia, continue to prohibit alcohol for religious reasons. In the United States there are still communities with a ban on alcohol sales. Weston is one of 17 communities in Massachusetts with such a ban. In 2003 there was the first attempt to change that rule after at least 25 years. It was not successful.

Most countries have laws against drunk driving, driving with a certain concentration of ethanol in the blood. The legal threshold of blood alcohol content ranges from 0.0% to 0.05% or 0.08%, according to local law.

Most countries also specify a legal drinking age, below which the consumption of alcohol is prohibited. In the US, the legal age in every state has been 21 since the passage of the National Minimum Drinking Age Act in 1984, which tied federal highway funds to states' raising their minimum drinking age to 21.

In small amounts, ethanol causes a mild euphoria and removes inhibitions. In large doses, ethanol acts as a central nervous system (CNS) depressant and causes drunkenness (at a blood ethanol content of about 0.1%). At higher contents, alcohol causes intoxication, coma and death. A blood ethanol content above 0.4% can be fatal, although regular heavy drinkers can tolerate somewhat higher levels than non-drinkers. Eight to ten drinks per hour is considered a fatal dosage for the average 120 lb person. One drink is equivalent to one shot of 80 proof liquor, one 12 oz beer, or one 4-5 oz glass of wine. The highest recorded non-lethal level of alcohol is 0.914%.

Alcoholism, the dependency on alcohol, is a major public health problem. Alcoholics develop a number of health problems, with cirrhosis of the liver being the most important one.

Excessive alcohol consumption during pregnancy carries a heavy risk of causing a series of permanent mental and physical defects in the child, known as fetal alcohol syndrome.

Ethanol is quickly absorbed into the bloodstream and reaches the brain. As a small molecule, it is able to cross the blood-brain barrier. The euphorizing effects of ethanol are probably due to its causing the release of endorphins, natural "feel-good" molecules.

The CNS depressant effect likely is due to ethanol's acting on the BK channels. A BK channel is a calcium dependent potassium channel. It has been known to act on GABA receptors, but this is probably just a secondary effect from activation of the BK channels. Its effect on GABA receptors is probably similar to the action of benzodiazepines such as diazepam. GABA is an inhibitory neurotransmitter, meaning it acts to slow down or inhibit nerve impulses. Ethanol increases the effectiveness of GABA acting through GABAA receptors. When used over a long time, ethanol changes the number and type of GABA receptors, and this is thought to be the cause of the violent withdrawal effects of alcoholics.

Ethanol also interferes with synaptic firing and causes the death of brain cells. This cell death is caused by an increased concentration of intracellular calcium which weakens the electrochemical gradient across the cell membranes. It is this gradient which is the motive force of membrane pumps and channels (cells, especially neurons, quickly die without proper membrane pump and channel function). There is also direct damage to cell membranes from free-radicals that are produced from alcohol metabolism.

Small amounts of alcohol do not act as a carcinogen. However, many studies have shown that large amounts of alcohol greatly increase the risk of developing a cancer. The strongest link between alcohol and cancer involves cancers of the upper digestive tract, including the esophagus, the mouth, the pharynx, and the larynx. Less consistent data link alcohol consumption and cancers of the liver, breast, and colon.

Upper digestive tract. Chronic heavy drinkers have a higher incidence of esophageal cancer than does the general population. The risk appears to increase as alcohol consumption increases. An estimated 75 percent of esophageal cancers in the United States are attributable to chronic, excessive alcohol consumption.

Nearly 50 percent of cancers of the mouth, pharynx, and larynx are associated with heavy drinking. According to mid-1980s U.S. case-control study, people who consumed an average of more than four drinks per day incurred a nine-fold increase in risk of oral and pharyngeal cancer, while there was about a four-fold increase in risk associated with smoking two or more packs of cigarettes per day. Heavy drinkers who also were heavy smokers experienced a greater than 36-fold excess compared to abstainers from both products.

Prolonged, heavy drinking has been associated in many cases with primary liver cancer. However, it is liver cirrhosis, whether caused by alcohol or another factor, that is thought to induce the cancer. In areas of Africa and Asia, liver cancer afflicts 50 or more people per 100,000 per year, usually associated with cirrhosis caused by hepatitis viruses. In the United States, liver cancer is relatively uncommon, afflicting approximately 2 people per 100,000, but excessive alcohol consumption is linked to as many as 36 percent of these cases by some investigators.

The liver contains a special enzyme (alcohol dehydrogenase) that breaks down alcohols into acetaldehyde, which is turned into acetic acid by the enzyme acetaldehyde dehydrogenase, and then yet another enzyme converts the acetate into fats or carbon dioxide and water. The fats are mostly deposited locally which leads to the characteristic "beer belly". Chronic drinkers, however, so tax this metabolic pathway that things go awry: fatty acids build up as plaques in the capillaries around liver cells and those cells begin to die, which leads to the liver disease cirrhosis. The liver is part of the body's filtration system and if it is damaged then certain toxins build up thus leading to symptoms of jaundice.

The alcohol dehydrogenase of women is less effective than that of men. Combined with the lower amount of water in women's bodies, this means that women typically become drunk earlier than men.

Some people, especially those of East Asian descent, have a genetic mutation in their acetaldehyde dehydrogenase gene, resulting in less potent acetaldehyde dehydrogenase. This leads to a buildup of acetaldehyde after alcohol consumption, causing hangover-like symptoms such as flushing, nausea, and dizziness. These people are unable to drink much alcohol before feeling sick, and are therefore less susceptible to alcoholism. This adverse reaction can be artificially reproduced by drugs such as disulfiram, which are used to treat chronic alcoholism by inducing an acute sensitivity to alcohol.

Consumption of ethanol has a rapid diuretic effect, meaning that more urine than usual is produced, since ethanol inhibits the production of antidiuretic hormone.

Overconsumption can therefore lead to dehydration (the loss of water). It is difficult to replenish the body's fluids using only alcoholic beverages. As large amounts of alcohol are consumed, the diuretic effect causes the body to lose more water than is contained in the beverage.

A common after-effect of ethanol intoxication is the unpleasant sensation known as hangover, which is partly due to the dehydrating effect of ethanol. Hangover symptoms include dry mouth, headache, nausea and light sensitivity. These symptoms are partly due to the toxic acetaldehyde produced from alcohol by alcohol dehydrogenase, and partly due to general dehydration. The dehydration portion of the hangover effect can be mitigated by drinking plenty of water between and after alcoholic drinks. Other components of the hangover are thought to come from the various other chemicals in an alcoholic drink, such as the tannins in red wine, and the results of various metabolic processes of alcohol in the body, but few scientific studies have attempted to verify this. Consuming a large amount of water is the best way to overcome a hangover.