I once asked this question to a teacher when I was studying to be an actor, and this is what he said: There are 2 main approaches to any sort of role. You either try to pick up on certain physical things, like the way a character walks, or where they tend to focus their attention (floor, past the people they're talking to, etc.) Once you have that down, you try to internalise these physical cues. The other way is to try and put yourself in the character's shoes and let the rest come naturally. For either of these approaches, familiarity with the character is crucial. Whether this means observing the mannerisms of drug users or knowing what they're experiencing while they're on whatever is irrelevant. The important thing is familiarising onesself with the character, whatever state they're in. He said that yes, some actors get high for scenes where they're supposed to be high, and/or to "improve" their performance, and yes, some will take drugs to better understand a character they play that takes drugs, some will get drunk in order to convincingly do sex scenes where they have no attraction to the other cast member. However, at least as far as the world of theater goes, most actors are sober for most of their performances.
This may be one man's opinion, but this one man happened to have a Master's in theater arts, years of experience in various roles on and off stage, and was familiar with the ins and outs of the business. He also told me a funny story about how it was not uncommon for theater stars to go across the street to the bar and slam down a few drinks during intermission, sometimes too many drinks. He told the class how Lawrence Olivier and one of his costars did this one night, and during act 2, one of the sober cast members was so thrown off she forgot her lines and yelled "You're drunk!", the response was something like "Wait until you see Mr. Olivier".