From Keith Humphreys:
Needle exchange and other programs that attempt to promote safer injection rely on behavioral change. Anyone who has been on a diet knows that behavioral change is hard to do lastingly and perfectly, even for people who are not addicted to a drug. It is therefore unsurprising that research using DNA samples from needles returned to exchange programs shows that most have in fact been shared.
What needle exchange does is make sharing less common in a population rather than eliminate such sharing altogether. With a harder to transmit virus such as HIV, that can be enough, but with an easily transmitted virus such as Hepatitis C, it isn’t. And these things snowball rapidly as easier transmission leads to higher prevalence in the drug using population, which creates more risks of exposure. Modelling research by Harold Pollack shows that programs that attempt to make injection safer can reduce population rates of HIV, but not Hepatitis C.
The reality is that in many cities a person who engages in extensive injection drug use is probably going to contract Hepatitis C. Stopping injection drug use entirely is thus a critical public health goal.
Now, he goes on to argue for opiate substitution. He should spend a little time reading this blog.