This isn't really true, pharmacologically. One way to measure the safety of a drug is the therapeutic index [1], basically the ratio of the lethal dose to the effective dose. It's basically how much of a margin of error you have when administering the drug before it'll kill you. Morphine has a TI of around 70, marijuana and LSD are > 1000, codeine is 20, alcohol is around 10, and heroin is 5 [2].
And even lower again is Paracetamol/Acetaminophen (Tylenol if you're in the US), with the fatal dosage being as low as 3 times the recommended dosage [1]. I can walk into a pharmacy here and buy enough of it to shut down my liver for less than a pound.
I get what you're saying, but if you're talking about overdosing, then I think it's fair to say Heroin is absolutely lethal. But if used responsibly (I know, I know) it's safe.
[1] http://en.wikipedia.org/wiki/Therapeutic_index
[2] http://www.americanscientist.org/libraries/documents/2006451...