This is a topic we avoid all too frequently. In fact, I believe its only mention on the site is in PokerCheat.com, where we discuss the double-edged sword about how booze makes a cheat more likely to cheat, but also makes him easier to catch.
But, what about where it affects your game? This seems like a straightforward point, and Lord knows I've been playing poker long enough to know this. But, one explanation for my recent loss was the fact that I was a little tipsy before the game even started. Not the annoying drunk stage that nobody wants to deal with, but certainly that inebriated stage that follows 3-5 drinks.
Understand where some of the points below may not apply to you specifically. Everybody handles the booze differently, and some people can't handle it at all. If you don't drink, you may enjoy the observations made in this article to help you against the players who do drink.
Levels of Intoxication
Let's start sober. If you don't drink, good for you. If you do drink, there's varying stages.
Now let's have a couple drinks. This is probably the best stage to be at, because alcohol does loosen the nerves. You likely haven't yet reached the point where you can't trust your own judgment, but you do feel more relaxed. When you feel more relaxed, you're likely to have an easier time following and responding to the flow of the game. I daresay that this is the ideal amount of alcohol to drink during a poker game (feel free to disagree).
Now let's have a couple more drinks. This is the stage that I was at in my game, and it's not where you want to be if your game is serious or even semi-serious. The players in my game were all of a high calibre, so having this many drinks was a mistake. I could tell as soon as the first hand was dealt that I had absolutely no sense for the pulse of the game and my place in the flow. Reserve this type of drinking for the less serious games.
Now let's have even more drinks. This is like playing poker in Las Vegas, where beers only cost your tip to the waitress. It's tough to pass up free beers, and in a night of poker, it's tough to pass up having plenty of drinks with your friends. Throw your intuition, judgment, and smarts out the window on nights like these, because it won't help your poker game. Reserve this type of drinking for games that aren't serious at all, or ones that don't concern you in terms of what you stand to lose. The only advantage is that alcohol makes some people more aggressive, and that's actually a valued trait in the game. But, loose-aggressive can't touch tight-aggressive, and you want the latter style when you want to make money.
If you don't have a plan and the booze is flowing, expect that you will reach the third level of intoxication. I personally like having a drink while I play cards, so ideally, I'm for the first level of intoxication; the one where you've relaxed your nerves but kept your best wits about you.