The short-term effects of alcohol in small doses are reduced tension and relaxation, but these effects are also accompanied by reduced inhibitions, decreased coordination and reaction time and memory problems - all of which put users at risk. Drinking too much too quickly or binge drinking increases these risks. In addition to the serious danger of alcohol poisoning, the depressant effects of alcohol can overwhelm the body’s defenses leaving users unable to move and think clearly which may lead to reckless actions that are unsafe or even lethal.
Alcohol is a depressant derived from the fermentation of natural sugars in fruits, vegetables and grains. These are brewed and distilled into a wide range of beverages with various alcohol contents.
Alcohol travels through your bloodstream and can damage your brain, stomach, liver, kidneys and muscles. Drinking alcohol as a teenager when the body is still developing can cause lasting damage wreaking havoc on the body and mind. Continued heavy use of alcohol leads to increased risk across the lifespan for medical problems such as cancers of the oral cavity, larynx, pharynx, and esophagus; liver cirrhosis; pancreatitis; and hemorrhagic stroke.
Many people are surprised to learn what counts as a drink. A standard drink is:

12 ounces of beer (about 5% alcohol)
8 ounces of malt liquor - beer with a high alcohol content (about 7% alcohol)
5 ounces of table wine (about 12% alcohol)
1.5 ounces (a “shot”) of liquor, like gin, rum, vodka, tequila, or whiskey (about 40% alcohol)

No level of drinking is safe or legal for anyone under age 21.
U.S. Department of Health and Human Services. The Surgeon General's Call to Action To Prevent and Reduce Underage Drinking. U.S. Department of Health and Human Services, Office of the Surgeon General, 2007.
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National Insitute on Alcohol Abuse and Alcoholism. Overview of Alcohol Consumption
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