There is a mysterious psychological connection between pain and pleasure. Just as the sensation of pain tells us to stop harm to ourselves and to avoid harmful actions, the sense of pleasure encourages the continuation of the action that delights the brain and pleasures the body. The relief from an existing pain leaves us not only with the normal feeling of having no pain, but also with a special sensation of pleasure and satisfaction from the absence of pain. Pain can activate internal mechanisms which not only suppress the pain itself but also create pleasure, and although such a reaction to pain is not normal, there are some people who find a masochistic pleasure in the pain itself.
Therefore, it is interesting to look into the neuro-chemical connection between the sensation of pleasure and pain: this connection is represented by the endorphin system responsible for the suppression of pain as it is for the sensation of pleasure. Endorphins are released naturally into the synapse, to block pain signals from arriving to the following nerve cells and to the part of the brain called the Corpus Striatum. Similarly, nerve cells release natural endorphins to monitor and regulate the activity of neurons that release dopamine - a neurotransmitter that stimulates the pleasure/reward center in the limbic system of the brain, which is responsible for emotions and sensations such as the sensation of pleasure.
The disruption of the natural endorphins regulation mechanisms (by introducing external opioids into the body) has two aspects: the positive aspect lies in the possibility to block the receptors from relaying extreme pain, for medical reasons), and the dangerous negative aspect – the primary reason for opioids' abuse – lies in the suppression of the natural control system and the intense artificial stimulation of the pleasure/reward center in the brain.
This paradoxical biochemical connection between pain and its prevention, and pleasure, is also responsible (in addition to factors already mentioned above) for the significant attraction to the misuse of opioids - especially heroin.
The same paradoxical duality existing between pleasure and pain can also be attributed to the behavior of opiate addicts. They consume heroin at first for fun, and later, after the creation of addiction, mainly to relieve the pain of physical and psychological withdrawal symptoms.