Lets look at some of the substances people might use to help get into the creative mindset, and examine why these aren't helpful. First, lets look at alcohol. I had a friend who used to often produce music while drinking. I tried to produce a peice of music with him once, and what came out was an incredibly sloppy peice of work. Towards the end of the session we were putting down ideas without any regards for where the project was going or wether they fit the context of what we were doing. Things like levels and EQing were all over the shop too. Coming back to the project later, most of it was unusable or needed serious time put into it to make the ideas pleasing in any way. By all means, have a beer while you are making music, just don't let it get to the stage where you've had 4 or 5 and you're trying to execute an idea that inevitably will need some finesse.
Next, lets look at smoking weed while you're making music. I've never been too big on smoking weed, but again I can highlight the example of friends that I know who produce music while stoned. Typically, they'll open a project they've been working on then light up a joint to help with their creativity. If they do manage to start work on something at all (often they will get distracted by whatever tabs are open on their web browser, or start listening to their mp3 collection instead) then their work goes off into tangent after tangent. They never stick to an idea, and they get distracted by a shiny new peice of software or something. The next day they wake up to an incoherent fractured mess. As with the last point on alcohol, by all means smoke weed if you enjoy it, just wait until you've finished making music. It can be a lot more fun to just chill and listen to music after a joint anyways.
Lets look at amphetamines now. It's no surprise that students will often use prescription amphetamines (used to treat ADD and ADHD) to help with focus while writing an essay. So the logic would go something like focus of that nature should help in writing music. This is something I do have experience with, and that might be relatable to some of you. I've always found writing music in this way will have one of two effects. Either you'll finish something, but because the amount of adrenaline in your system it tends to lack a bit of feeling or emotion, or you become too obsessed with the perfect excecution of your idea and it's never good enough. Going back to the example of a student using amphetamines to focus on writing an essay, it would be like getting stuck trying to write the perfect sentence before moving on to another part.
In truth, I think that being in the zone creatively requires you to be relaxed AND focused. You have to be doing something that is challenging enough that it really engages you. However at the same time, you can't be too conscious of what it is you are doing or which direction you are going in. You need to be relaxed and just disconnected enough that it leaves the door open for happy accidents and creative exploration. If you are too conscious of what you are doing you tend to have a predetermined idea of how everything needs to be, when rather you need to be relaxed enough to let your creativity dictate that direction. This balance between relaxation and focus I dont believe is best acheieved while under the influence, as any substance you do use will likely push you too far in one direction or the other. Futhermore the use of any of these substances can have a longer term impact on your mental health, prohibiting you from being optimistic about your creative prospects, a key point in remaining creative and consistently outputting creative content.