The discussion about tolerance is interesting because there are documented cases of habitual heroin users surviving extremely high doses, especially if the substance is not pure. However, if we are talking about a truly lethal dose, for a person with no tolerance, as little as 250-300 mg of pure heroin can cause a fatal overdose, while those with developed tolerance may require significantly higher amounts. The most important variable is not just the absolute dosage but also the combination with other central nervous system depressants, such as benzodiazepines, alcohol, or synthetic opioids, which drastically enhance respiratory depression and reduce any margin for error. Even though heroin alone might have some level of unpredictability, combining it with other substances makes the outcome much more reliable, ensuring unconsciousness well before respiratory arrest occurs.
Regarding suffering, death from heroin overdose is caused by respiratory depression, not acute physical pain. The primary mechanism is the progressive slowing of breathing until it completely stops, often accompanied by loss of consciousness in the early stages. If someone experiences sensations of suffocation, it could be due to respiratory spasms during the transition from hypoventilation to apnea, but this depends on individual physiological responses. In most cases, with a sufficiently high dose and especially with the addition of synergistic drugs, unconsciousness occurs before the critical phase, eliminating any perception of suffering. The idea that overdose leads to a conscious struggle does not account for the fact that heroin, as a powerful opioid, has strong sedative and analgesic effects, preventing a clear awareness of one's condition in the final moments.
If we consider a dose that is certainly lethal without the need for additional substances, the amount would be around 2 grams of pure heroin for a person with no tolerance. At this level, despite individual variations, the risk of survival becomes negligible, especially when administered intravenously, which ensures immediate absorption and a direct effect on the central nervous system.
For a person with very high tolerance, such as a long-term heroin user with heavy habitual use, the lethal dose can range between 5 and 10 grams of pure heroin, depending on the administration route and the body's ability to metabolize the substance. However, at these levels, even for someone with a developed tolerance, the margin for error is extremely small, and any variation in the purity of the substance could lead to a fatal outcome.