I’ve never been a drug addict myself (I’m glad to say) but this description of addiction from Eric Ambler’s The Mask of Dimitrios paints a very convincing picture of how addiction can take hold:

The process is always roughly the same. It begins as an experiment. Half a gramme, perhaps, is taken through the nostrils. It may make you feel sick the first time, but you will try again and the next time it will be as it should be. A delicious sensation, warm, brilliant. Time stands still, but the mind moves at a tremendous pace and, it seems to you, with incredible efficiency. You were stupid; you become highly intelligent. You were unhappy; you become carefree. What you do not like you forget; and what you do like you experience with an intensity of pleasure undreamed of. Three hours of paradise. And afterwards it is not too bad; not nearly as bad as it was when you had too much champagne. You want to be quiet; you feel a little ill at ease, that is all. Soon you are yourself again. Nothing has happened to you except that you have enjoyed yourself amazingly. If you do not wish to take the drug again, you tell yourself you need not do so. As an intelligent person you are superior to the stuff. Then, therefore, there is no logical reason why you should not enjoy yourself again, is there? Of course there isn’t! And so you do. But this time it is a little disappointing. Your half a gramme was not quite enough. Disappointment must be dealt with. You must wander in paradise just once more before you decide not to take the stuff again. A trifle more; nearly a gramme perhaps. Paradise again and still you don’t feel any the worse for it. And since you don’t feel any the worse for it, why not continue? Everybody knows that the stuff does ultimately have a bad effect on you, but the moment you detect any bad effects you will stop. Only fools become addicts. One and a half grammes. It really is something to look forward to in life. Only three months ago everything was so dreary, but now … Two grammes. Naturally, as you are taking a little more it is only reasonable to expect to feel a little ill and depressed afterwards. It’s four months now. You must stop soon. Two and a half grammes. Your nose and throat get so dry these days. Other people seem to get on your nerves, too. Perhaps it is because you are sleeping so badly. They make too much noise. They talk so loudly. And what are they saying? Yes, what? Things about you, vicious lies. You can see it in their faces. Three grammes. And there are other things to be considered, other dangers. You have to be careful. Food tastes horrible. You cannot remember things that you have to do; important things. Even if you should happen to remember them, there are so many other things to worry you apart from this beastliness of having to live. For instance, your nose keeps running: that is, it is not really running but you think it must be, so you have to keep touching it to make sure. Another thing: there is always a fly annoying you. This terrible fly will never leave you alone and in peace. It is on your face, on your hand, on your neck. You must pull yourself together. Three and a half grammes …