Respiratory Arrest. An overdose of heroin shuts down breathing, and depending on how severe the overdose, it can also shut down central nervous system.
There was long while ago, a case of Parkinson's Disease-like symptoms after heroin use. It turns out that improper Demerol synthesis (a fully synthetic opiate as opposed to heroin, a quasi-synthetic one), results in a byproduct called MPTP. I'm not sure of the specifc action of MPTP in the brain, but it really, really fvcks it up. Whatever lab tried to cook up Demerol, wasn't too careful.
On a side note, the common myth that Ecstasy causes Parkinson's Disease seems to have resulted from the media's confusion between MDMA (ecstacy) and MPTP.