There is another take on that subject that I read about a few months ago in the Baltimore (MD) Sun. The article was about medical examiners in many cases not being able to determine an actual cause of death in drug related deaths. They can determine that a certain drug caused the death but they have no way to tell for certain if it was homicide, suicide, or accidental according to the article, and many are reluctant to make an official ruling.. A person can shoot up, go into some kind of stupor and someone could give them another dose or two to deliberately cause the overdose for instance. When police find a body of an obvious drug abuser they rarely do lengthy investigations.
More good reasons to stay away from narcotics I guess.
I couldn't locate the article but this link is a forum where I think some of the points from the article are discussed and they use some quotes from the article.
http://talk.baltimoresun.com/showthread.php?t=255375