Alcohol is a central nervous system depressant. In small amounts, most people experience sensations of relaxation as alcohol "depresses" brain activity. As more alcohol is consumed and levels of alcohol within the body rise, certain parts of the brain are affected, and declines in functioning occur.

The cerebrum controls higher functions, such as vision, emotions, reasoning, and recognition. When alcohol depresses the cerebrum, inhibitions are lowered, judgment is impacted, and impairments occur in vision, movement, and speech. These effects have resulted in alcohol as a "social lubricant," as individuals frequently are less inhibited and more likely to engage in conversations and disclose more than they would when sober. Remember, judgment is also impaired, and one may be embarrassed by his or her actions and conversations under the influence. This occurs at blood alcohol levels of .01%-.30%.

The cerebellum coordinates movement; thus, when depressed by alcohol, the effect is impaired coordination, slowed reflexes, and impaired balance. These are some of the effects that may be evaluated when an individual is asked to walk a straight line, balance on one foot, etc. These impairments occur at blood alcohol levels of .15%-.35%.

The medulla controls basic survival functions, including regulation of the cardiac and respiratory systems. When considerable amounts of alcohol have been consumed, the effect can be toxic and deadly. When the medulla is depressed, an individual's heart rate and blood pressure can drop to dangerous levels, and respirations will significantly decrease. Alcohol poisoning can result in death when the heart rate and breathing cease. These dangerous effects can occur at blood alcohol levels as low as .30%.