Wikipedia is your friend...
According to Wikipedia, "drug" is a bit vaguely defined. You have stuff ranging from the humble aspirin to all these modern antibiotics: medicinal drugs.
When it comes to aviation many people are surprised to find that many medicinal drugs are illegal! The drug puts you in a condition of unfitness to fly, simply put.
Then we come to the fun stuff: recreational drugs. Some of the really enjoyable ones are unfortunately illegal, which does seem rather a shame. Okay, cannabis can rot your brain and ecstasy (MDMA) turn you into a vegetable but... So the nanny state tries to save us from ourselves by making them illegal. As far as I know there's no rehab program for cannabis users who are pilots but I could be wrong about that. As to ecstasy there was just a report published about a PPL who took some and then went flying, killing himself and his young passenger.
Alcohol is a legal recreational drug but not everywhere and always. Plus there are usually limits to its use so that simply taking too much can see you arrested.
Alcohol is also a poison, hence the term "intoxicated". which literally means "having ingested poison". Every so often we get to read about someone who checks out after proving that he can "chug" a quart of vodka. That puts the body into a footrace between nausea (a protective reflex) and death from alcohol poisoning. Usually our hero barfs all over himself before passing out, to general merriment and no one permanently the worse off but every so often the alcohol is kept down to command a general shut-down of first higher brain functions but later all those boring things such as breathing that keep us alive.
Alcoholism is accepted nowadays as a disease with stages and one "marker" can be alcohol tolerance. Basically a heavy drinker can drink more and, while still impaired, function better than a non-drinker with the same blood alcohol level. In the terminal stage of the disease alcohol is necessary for proper functioning; this is when you get the snakes crawling up the walls and the "shakes" until that first drink to start the day with.
I knew a guy who could put away a quart of Scotch an evening but do a pretty good job of flying the next morning early. He still lost his job, though. It was just that in those days and in that environment we had all been trained to look the other way for the most part. I tried to intervene but got nowhere with him.
In an ideal world, like the one dreamed of by the Nazis, anyone with a disability would be quietly done away with so that we would have a godlike general population. Instead we try to find ways of allowing people with flaws, and that includes most of us beyond a certain age, to still work and contribute to society, including alcoholics.
An odd thing is that you can end up flying again but you will still be an alcoholic, just one who doesn't drink! (Think of it this way: I am a raging heterosexual but that doesn't mean I am going to have sex during the flight. I have learned to control my male urge, always present, to have sex.) There are ways to try and teach people to control the urge to drink as well that have been proven to be effective. The rationale is that it is much better, much safer in the long run, to have rehabilitation programs in place if the alternative is a pilot hiding his problem and going untreated.
Everytime there is an incident like this one you will get a typical response from some that anyone with a drink problem should be tossed out of aviation without a second thought. The funny thing is, I usually imagine that same sort of responder then having another snort before going on to fulminate about the next social problem that shows up on his screen, homosexuals allowed to marry and adopt children, perhaps. At best that's just nostalgia for an age when we went for simple solutions and kept problems decently out of sight. "Times have changed and we must change with them." Well, that is what my 19 year-old daughter keeps telling me, anyway!