There are moments in this book that grab you; the fine detail of the mundane misery and depths of addiction. However, the major problems with this book are those of pace, trajectory and narrative.
Put simply, it is phenomenally one-paced; there is never a sense of urgency or reflection. Everything simply drifts on at much the same speed.
There is no trajectory to the storyline or characters - while this may be partly deliberate, it is also an error. Characters pop up and disappear with alarming regularity; none of them mean anything to either the reader or other characters; none of them are going anywhere or have been anywhere (in any sense); they are simply there, and then gone. The whole book is aimless. In addition, all the characters are defined by a narrow range of traits, and so start to meld into each other.
Narrative? Doesn't really have one. It has about 120 individual set-pieces or vignettes. Each one is fine in itself; but it becomes a real drag to keep reading tiny unconnected sets of observations that never gel.
Overall, this strikes the reader as something written by someone who has experienced a number of things, but had no idea how to communicate them coherently. Instead, it reads like dozens of short creative writing exercises about the same subject, which just happen to be published under the same cover. As such, there is no real reason for the reader to care, or to keep reading. And it is this, that makes the book less absorbing as it ought to be.