A way of inducing an altered state of consciousness, just like sleeping and meditating. This isn't problematic for the "brain in vat" scenario.
Sleeping and meditating are both internal to the brain, but alcohol only becomes internal to the brain if it has some physical access to the brain.
For the brain in a vat theory to work, the computer controlling the simulation would have to monitor the amount of alcohol that is in the bloodstream of the simulated being, and adjust the alcohol in the vat accordingly?
IF a consciousness was housed in a black box (a brain, but we're ignoring it's internals) AND that black box had inputs connected to a computer that can send signals to the black box that it WOULD interpret as signals of sight, sound, touch, hunger, intoxication, etc.
THEN how is one suppose to conclude that they are not just a consciousness in a box because they 'feel' drunk after they 'feel' they've ingested alcohol.
There's no need to monitor alcohol levels because the alcohol hasn't been proven to exist!
Because if the world out there doesn't exist, these things shouldn't effect the way you think. If something is simulating the world for you, you have to assume every time you drink, that thing alters the way you think somehow, which means it can access your actual real brain, whatever it is.
Then that brain of yours, even if it's outside the world, must be so similar to all the other brains you see, which are effected similarly by those same substances.