There is a tendency to view these things as black and white. But with MH issues and addiction, they almost never are.
I don't think addicts do have a moment where they think 'I can't cope without this, therefore I am an addict, which is bad, and therefore I should stop doing it', I think they're probably more likely to (in the v early stages of addiction) think, 'this is really painful, but if I just have this helping hand, I will manage today, and then tomorrow's another day' etc. etc. At the early stages of the addiction, their 'substance of choice' for want of a better word is always their best friend, it's a positive thing in their lives (in their eyes), it makes their life better and keeps them on the 'straight and narrow' (yes I know that phrase is completely oxymoronic in this context), or so they think. Then they have it more and more, and then it's the only thing in their lives and is anything but positive.
I had a shortish bout of moderate PND, and even though I had lots of help and support and a loving family and a nice life, I would have given a lot, to not feel like that. What I went through is not even a drop in the ocean compared to the horror that is the life some of these people lead.
I have a friend who is not an addict, but he could very easily be, he just doesn't have an off switch, when he drinks, he just doesn't experience that moment I have, where I think...mmmm getting a bit squiffy now, think I'll have a glass of water and a sleep....he just doesn't get that, he just ends up rat-arsed. He's not deliberately irresponsible, he just doesn't have the moment that I have, where I make a decision. Some people are just wired that way. I think it must be a thousand times harder for people wired that way, to have any sort of self-control or discipline or whatever, over potentially addictive substances. I'm not even saying it is 100% out of their control, I'm saying it's a lot harder for some people to control than others, so I wouldn't really like to judge them, not knowing how hard it is for them. FWIW, I would keep a real close eye on this friend if he was for whatever reason on prescribed addicted drugs as Maryz describes.
Me, I'm a bit like that with butter and mayonnaise, hence being a bit squidgey around the edges, which is still frowned upon, but more socially acceptable than being an alcoholic or drug addict I suppose.
D