Alcohol commonly refers to Ethanol (C2H5OH). Human consumption can be a very vile thing. It makes people inadvertantly end up with their tongues in the mouths of people they swore they'd stay away from before they opened that can of Budweiser. It makes people spew brown liquids out of their mouths, and into the sink (if they're fortunate), trash basket (if they're less fortunate), or all over the floor (if they're my brother). It makes people get into fights with each other for no apparent reason. It can cause permanent anterograde and/or retrograde amnesia (as in Korsakoff's syndrome).

There are plenty of reasons not to drink, but, that said, it's also a lot of fun.

Harm Reduction
Proper harm-reduction depends on several matters. First, make sure you can't drive. If your judgment is impaired enough when completely trashed that you might consider it, give someone else the keys. It's an easy thing to do, and it could just save you your life.

Second, take proper supplements. A B-complex vitamin helps prevent deficincies in essential nutrients that can lead to cognitive deficits. The aforementioned Korsakoff's Syndrome is shown to result directly from vitamin B1 deficiencies. N-acetylcysteine is a powerful antioxidant that removes stress on the liver, and reduces the interaction between alcohol and acetaminophen (Tylenol). Be careful with it, however. High doses can cause excitotoxicity and thus brain damage themselves, negating any positive effects.
See R. Orasas et al, N-acetylcysteine attenuates alcohol-induced oxidative stress in the rat, PMID 12508366 and E. A. Carter, Enhanced acetaminophen toxicity associated with prior alcohol consumption in mice: prevention by N-acetylcysteine, PMID 3828067 for more on the possible benefits of N-acetylcysteine.

Third, be aware of how drunk you are, and how long you'll stay drunk. Drinking on an empty stomach is, as any frequent drinker knows, going to get you much drunker than drinking on a full stomach, especially if you're taking shots. Alcohol metabolism rates begin a steep rise just before midnight, and at 6 AM, peak at four times the rate of metabolism at 6 PM. Thus, the later you start drinking, the less time it will take you to sober up.
For a graph of alcohol metabolism rates versus time of day, see