Ethanol acts as a drug affecting the central nervous system. Its behavioral effects are a result of its influence on the response in the nervous tissue and not on the muscles or senses themselves. Alcohol is a depressant, and depending on dose, can be a mild tranquilizer or a general anesthetic. It suppresses certain brain functions. At very low doses, it can appear to be a stimulant by suppressing certain inhibitory brain functions. However, as concentration increases, further suppression of nervous tissue functions produce the classic symptoms of intoxication: slurred speech, unsteady gate, disturbed sensory perceptions, and inability to react quickly. At high concentrations, ethanol produces general anesthesia; a highly intoxicated person will be in a coma like state and very difficult to wake. In extreme cases, if the alcohol concentration is high enough, it will inhibit basic involuntary bodily functions such as breathing and can cause death.

When an alcoholic beverage is consumed it passes down the esophagus through the stomach and into the small intestine. Although a small amount of alcohol is absorbed into the bloodstream through the mucous membrane, that vast majority of alcohol enters the bloodstream through the walls of the small intestine. Alcohol is water soluble and the bloodstream rapidly transports the ethanol throughout the body where it is absorbed into the body tissues in proportion to their water content.

Ethanol is greatly diluted by the body fluids. For example, a 1-ounce shot of 80-proof whiskey, which contains 0.4 fluid ounces of ethanol will be diluted in a 150-pound human, producing somewhere in the neighborhood of an 0.02% blood alcohol concentration. With a user that is smaller with say one half of the water weight in his or her body than the individual in the prior example, that same 0.4 fluid ounce of ethanol would likely produce an alcohol concentration at or near 0.04%.