Discussion on legalization of controlled substances like heroin is fraught with bias and misinformation. It's a discussion worth having, but clouding it with contradictions makes an already difficult topic that much harder to address.
pstuart didn't say if heroin had been pure that everything would be fine, just that the user might have a chance. And you definitively have a greater chance of surviving alcohol addition if you don't die from a single bottle[2].
[1] http://www.slate.com/articles/health_and_science/medical_exa...
[2] http://www.ibtimes.com/last-call-poor-indians-continue-die-a...
While alcohol and tobacco might have a larger "therapeutic index" then heroin, regular use can and does damage many peoples' bodies. Regular use of opiates causes constipation and a few other minor side effects. They aren't really comparable.
The main reason for overdoses on opiates is related to mixing with other drugs (alcohol and benzos). After that is the dosage amount. If patients always knew exactly how much they were using, like they would less likely accidentally use too much.
Another cause for overdoses is when a patient discontinues the medication, then resumes usage assuming their previous tolerance levels. If these medicines were legal, far fewer people would need to stop using in the first place, further reducing accidents.
The other health issues with opiates are a result of unhygienic IV use. If needles were readily available, and medicines pure, there would be less health issues arising. Instead, people reuse their needles many, many times, and users may often end up injecting all sorts of materials into their blood.
http://www.abc.net.au/news/2009-12-19/swiss-drug-addicts-giv...