What does it do? In my experience it can completely strip energetic protections/boundaries off and let other beings into your energy field: ghosts, mainly, and ghosts that were often alcoholic in life (they are also abundantly found in bars, BTW), but also what I'd call "entities of addiction". Sorry I don't have another name for them.
My guides tell me it also preserves - as in freezes or stagnates - or even lowers your energetic vibration, kind of like sugar does. This is how we plant people make things like flower essences: we use the alcohol in the preparation to stabilize and "freeze" the energetic vibration of the flower. If you drink alcohol a lot, my guides tell me it'll lower your vibration to that of the alcohol.
Part of what goes on, too, is the intention with which the alcohol is made. Meaning mass-produced alcohols (beer, vodkas, etc.) made with the intention of selling lots and lots of cases of beer or bottles of vodka will be more addicting/lower vibration than some thing like an ages-old, family owned artisanal whiskey or wine that's made with a lot of reverence, tradition and care. Of course, you can make your own alcohol (within the limits of the law) with the same intention, care and reverence and it will have a higher vibration.
Having a teensy little bit once or twice a year is okay, especially if it's ritualized. For example, I might actually drink a teeny bit of whiskey once in a great while for my Ancestors or as a way to connect with them. I usually give it to them on as an altar offering, though.
I've put a link to an interesting article about this subject in the comments, too. This one has made the rounds for a year or so, I think, and it's a pretty good explanation of how entities attach to people who partake too much.
Next week I'm going into how to use the natural elements (earth, wind, fire, water) to help cleanse your energy field. A much nicer topic!