Read this. There are also articles against this one but I think it makes a strong case. has the PR references as well.

http://www.huffingtonpost.com/johann-hari/the-real-cause-of-addicti_b_6506936.html

an interesting fact that junkies that addicted to injective type of drugs has also an addiction of injecting the needles in their veins.

which might also be connected with self-harm..yet again lack of emotional connection