For a long time, the patho-physiology of the drug heroin on the human body was not understood completely. Though heroin is one of the oldest drugs that have been abused by men, why it produces the typical very strong feeling of euphoria was not understood at that time. Another point that seemed elusive was about people wanting to take higher amounts of heroin after some period. It was only when heroin abuse deaths became such a grave problem that the medical experts began seriously looking into the issue.

The whole secret lay in the fact that heroin locates itself in the immune system and masks itself as one of the chemicals already present there. Dopamine in the human brain is responsible for the feelings of pleasure. Heroin ensures that the production of dopamine is heightened in its presence. Due to that, the person feels happier, a feeling we commonly know as the heroin addiction high.

But as the person takes heroin for some time, the body becomes immune to it. The dopamine secretion becomes static. The body does not get that euphoric feeling that it was used to. That is the reason why the person goes for a higher dose of heroin. This higher dose ensures the secretion of dopamine once again, and the heroin high happens again.

After regular intervals, the heroin addict will need to do this. The danger is that the person will never know when the amount of heroin has gone more than what the body can sustain. Such a situation can lead to death of the person even. This is what is known as a death due to heroin overdose.