The psychological addiction that intravenous drug users develope not to the drugs themselves but to the act of injecting them. Sort of like some weird placebo but with an effect all its own. Some drug users will simply inject saline or water to get this sort of rush. Its almost masochism, but probably has more to do with interfering with your body's natural systems and playing with something as important to life as your own blood. After all, its always more entertaining when its dangerous, right?