Drunk
Drunk is a condition that occurs after drinking alcohol. Beginning with "tipsy," if you continue to consume more drinks in quick succession you will become "drunk" and then eventually "totally smashed". Your ability to function in-game is severely impaired, including random changes of direction while walking/running. Additionally, your typed messages will be altered to have a drunken effect; your "S"s often become "SH"s and "...hic!" is added to the end of your lines on occasion (Example: What you shay might come out like thish ...hic!).
Gameplay effects
Getting a character drunk lowers the displayed level of enemies as they appear to the drunken player. The apparent level of all hostile targets (both NPCs and other players) goes down by up to 5 levels while "completely smashed", and slowly returns to its actual level as the character sobers up. It is important to note that while the level appears lower, it is actually unchanged; if you are not careful, you may find yourself fighting enemies much higher level than intended!
Movement is also impaired, as you veer sideways quite a lot when moving forwards. When flying, drunken players may veer to the left or right, and pitch up or down. Drunk-flying players will often find themselves flying directly upwards into the air or diving down into the ground. Swimming is definitely a bad idea; if you try to swim to the surface to get more air, you may find yourself swimming down instead, increasing the odds of drowning.
If a character consumes too much alcohol they will be afflicted with 
[Drunken Vomit], stunning them for a few seconds while green vomit spews from their mouth.
Sobering up
Drunken players will slowly become sober over a period of time, with automatic emotes expressing their increasing sobriety. As with getting drunk, drunken players will sober up by degrees; the effects of their intoxication will wear off step by step, with visual and movement disruptions becoming less pronounced.
Generally, the only way for players to become sober is to wait for the effects of the alcohol to expire. However, there are some exceptions. Players can drink certain beverages such as 
[Starfire Espresso] to sober up faster, with each cup reducing the player's inebriation by a reasonable amount; it may therefore take several cups to remove the effects of serious intoxication. Players can also cause their characters to sober up by allowing them to die; while not an ideal solution, it can be preferable to simply waiting when sobering beverages are not available.
Sobering beverages include:
- [Starfire Espresso] - consumable, made via cooking
- [Kafa Press] - re-usable item with 10-minute cooldown
- [Cup of Kafa] - consumable, one-time only quest reward
- [Keenbean Kafa] - re-usable item with a 10-minute cooldown, only usable in Kun-Lai Summit
Graphics effects
For supported graphics cards, the more drunk you become the more your screen will blur around the edges until it becomes nearly impossible to see. Players who desire to disable the blurriness can run the following command in their chat input, excluding the quote marks: "/console ffxGlow 0". (To re-enable, simply change the command to "/console ffxGlow 1".)
The glow effect requires the use of video card shaders to be enabled (fixedFunction = 0). If the graphic effect is missing, try typing these in your chat window first:[1]
/console ffx 1 /console ffxglow 1 /console fixedfunction 0 /console gxrestart
References
Patch changes
Patch 4.0.1 (2010-10-12): The screen blurring effect is now native to the client. (Full Screen Glow Effect video option was removed) 
Patch 3.0.2 (2008-10-14): A "Drunken Vomit" feature was implemented. This causes the character to "vomit" upon drinking too much alcohol. 
Patch 1.6.0 (2005-07-12): You no longer spout profanity when talking about sitting while drunk. - Patch 1.3.0 (2005-03-07): The Drunk effect has been greatly enhanced.
Patch 0.10 (2004-09-18):
- You weave when you walk if you are drunk.
- Drunkenness is saved when you log out and fades over 15 minutes.