most overdoses aren't overdoses. You've simply gone into different state of consciousness. Opiate users need to be taught that an overdose is when the breathing of the person is very slow and shallow, erratic, or has stopped.
Passing out from heroin is a common occurance, called going on the nod. Simply because your unable to rouse someone from being on the nod doesn't mean they're dying or overdosing.
In fact true clinical signs of opiate overdosing are very rare in regular users. Yes Opiate naive people who take a large dose of heroin can obviously go into overdose and die but the real factors for ovedose are as follows:
- Recovering user who, after a break, resumes their habit, taking a dose they were comfortable with. This is highly dangerous. There is evidence that the environment plays a factor in how the body metabolises the opiate. Taking drugs outside ones usual environment usually lowers one tolerance thus making it easier to succumb to a large dose.
- Alcohol and or Benzodiazepines . The vast, vast majority of opiate overdoses are the result of poly drug use that cause powerful interactions. Its really really dangerous to drink and shoot smack. It's one of the reason why heroin is outlawed in a sense. People perceive that heroin is the root cause when in actual fact it's the other drugs that are primary cause of the death.
- fast metabolisers
There are people who, after eating codeine and alike can metabolise a very large percentage of the codeine into morphine. See for most people your only metabolising a small percentage of the codeine into morphine (hence why the discussion in CWE to drink grapefruit juice and what not before taking codeine, to increase the amount metabolised). These people need to be careful about their opiate use.
Have a read from
The Consumers Union Report on Licit and Illicit Drugs Chapter 12:
The heroin "overdose" mystery and other occupational hazards of heroin addiction
tldr: basically heroin for regular users is really hard to overdose. Read the papers. Very informative. Basically it says that one of the cuts, commonly used with heroin (quinine) could be involved or, as I've indicated its polydrug combos and lazy authorities not bothering to actually investigate the cause of death properly.