Drugs are an important part of Disco Elysium, even if you choose not to take them. As the latest devblog from Robert Kurvitz over at Estonian indie studio ZA/UM points out, the full-sobriety challenge run is "favoured by our writers" and, "After all, there is no temptation without abstinence." Kurvitz then goes on to point out the many ways Disco Elysium has of tempting you.
Here's one: drugs make you better at things. Each of the four stats is connected to a drug, with amphetamines temporarily improving your motorics, for instance. That means it also improves every motorics skill and the maximum cap that skill can be raised to. An hour later the effect wears off, but if you took the opportunity to spend a point to boost a skill to the temporary maximum that point isn't lost. As Kurvitz says, "this started out as a bug, but we kept it because testers liked it."
Something I hadn't realized while playing is that though legal substances like booze and cigarettes are always visible in the world to tempt you, illegal substances are hidden until after you try some acquired another way (through dialogue, for instance). Stashes of speed and pyrholidon, Disco Elysium's made-up anti-radiation drug which improves your psyche stat, only become visible once you know what to look for.
If you're playing Disco Elysium already and aren't worried about some minor mechanical spoilers, like the effect of certain thoughts you can equip in your Thought Cabinet, this deep dive into drugs is worth a read. Kurvitz also notes that the brand names on substances have no effect because, "Just like in real life, brands do nothing."
Since drugs give mechanical bonuses (and also have substantial negative effects, to be fair) it's a wonder Disco Elysium made it past the Classifications Board here in Australia. A quick search of the Classification Database doesn't show it, perhaps because it hasn't been submitted for rating. Several members of the Board have been vocal about believing the rules they're obliged to follow are overdue for change, but without the cooperation of our lazy and conservative state governments it won't happen. In the meantime, I guess everybody is looking the other way—easy to do when a game is PC-exclusive and only available online.