This really isn't the issue that I was raising. Heroin is spectacularly dangerous and easy to OD on. Simply botching a little and taking 1.5 or 2x the "normal" dose can cause death. This isn't akin to taking 4 aspirin instead of 2, or smoking twice the meth (lots of nasty addicts do meth for weeks on end anyway, horrible).
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2967031/
The fatality statistics for Heroin are unrelated to long-term toxicity, but rather the incredible ease with which it can accidentally (or intentionally in the case of suicide) kill.
http://heroin.net/about/vital-heroin-statistics/
In short, heroin by a quite sizable order is dramatically more likely to kill an average abuser than any other common drug. About the only things comparable are other pharmaceutical products of similar natures, but even that isn't truly equal. A heroin abuser might buy a particular type of heroin for a period of time, and he could either switch dealers or the dealer could get a different supplier, and end up dying because he/she gets something a bit stronger than normal.