As i noted before, it's not just the concentration of heroin that causes overdoses, it's the combination of drugs or other substances in the heroin which cause the overdose. I think me and you, psy, may have a slight misunderstanding.
There are two types of interactions that cause heroin fatalities, and i will use layman's terms for this: metabolic - or liver, and effect-doubling on the brain itself.
The liver has certain enzymes, and pathways, for specific groups of substances. When a certain enzyme is preoccupied with breaking down a substance, it looses it's ability to break down others....you're body's reserves are preoccupied with something else. So for example, one common interaction is grapefruit and a few benzodiazepines: valium, ativan, xanax.. benzos and grapefruit are both processed by (i think) CYP-3A4. when consume both at the same time, your liver cant process either as efficiently and the effects of either substance is multiplied - eat a grapefruit with a xanax and you will get just as messed up as eating 2-3 times the dose without the grapefruit. same applies for Zantac and lexapro. take both, and you will have toxic levels and fluctuations of both.
another type of interaction is the doubling of effects, not through metabolic interactions but through simply doubling the effect on the brain itself. this is obvious - combine a depressant with another depressant, you will get doubly depressed. combine any of the following: opiates, alcohol, barbiturates, dissasociatives, deliriants, qualudes, or benzos, and the effect of each would exponentially enhance the other. One beer and a benzo is just as much as six times the dose of either individually - psychologically. BUT, the effect of either on the heart and lungs is uncertain. people have died from a beer and a benzo before, because even though they werent too "fucked up", their heart and breathing rate sank at some point to a dangerous level for a period of time. Same applies to opiates. you mix a beer or two with a shot of heroin or some oxy, and you're liable to make your heart slow down to dangerous levels.
Combine the fact that people who use heroin also use a variety of other substances, and the fact that heroin itself contains unknown substances which can have unknown effects on the metabolism, and you are injecting it directly into the bloodstream, and you have a dangerous combination. If you take a long acting benzo like librium for example - and shoot heroin a day later, you can die - of a heroin overdose. You might not think you're even mixing the two, but you are, because different substances also have different ranges of effects, half-lives, and metabolic pathways. It is true, most people who die of "overdoses" were in fact using the same amount they normally do. but the combination of that plus another substance caused their liver to stop working as efficiently, their tolerance went down temporarily, the concentration of whatever substance went up in their bloodstream, and they die. They die of an overdose, caused by an interaction. the interaction itself does not kill - it's the changes in liver chemistry that in turn effect concentration in blood which in turn causes a deadly effect.
Just to be clear, i'm not against drugs, but i'm not for them. i went through a period myself when i did quite a bit of drugs. i never touched heroin, crack, and meth, but i did almost everything else including some exotic stuff like the 2C's and DMT. i'm actually pretty stoned right now...nothing wrong with weed i think...which explains the crappy writing. I just think that people must be more informed and stop being so ignorant as to take one side or another. We have a shining example here, the person who started the thread, of someone who takes side. It's a fact, we get lied to alot by the government and other institutions about drugs. dare is bullshit. so people try weed, do a little research, and find out they were lied to about weed. they start smoking weed. then they get curious about other stuff, they figure "they lied to me about weed, they probably lied about everything else". so they do coke, or pills, and see it's not that bad (at first) and begin to take a nonchalant attitude towards it. eventually that nonchalance leads to drugs like meth, crack, heroin, and related drugs. {note: another thing causing the nonchalance is parents giving pills to kids, which then leads to kids taking whatever pills they feel like, but thats another dimension for another conversation]. You cant take sides like that. you need to do you own research, and find out for yourself what you're getting into from unbiased sources. you need to have limits, boundaries. for me, the limit of safe drug use is at moderate use of marijuana, alcohol (not liquor) and an occasional psychedelic tryptamine. other people might set that limit higher, but that should be an INFORMED risk. what bothers me the most is the person in question used the example of the individuals he met as an informative experience towards the safety of heroin. it's not. nor is the example of a junkie at rock bottom. the information should come from an objective source, not from subjective experience.