The Mechanism of Addiction
The two primary markers of addiction are dependence and tolerance.
Dependence occurs after a period of using heroin (or another opiate.) The brain’s function is adapted to its presence. The drug has been inhibiting the release of various neurotransmitters, and when the drug is taken away, the neurotransmitters are rapidly produced again. The sudden chemical imbalance in the brain leads to withdrawal.
Tolerance is the user’s progressive need to have more and more of the drug in order to feel the same effect. The reason for this seems to be that when the drug abuser’s brain is constantly exposed to a drug, it begins to "fight back," releasing more of a certain neurotransmitter, for example, to counteract the effects of the drug. A user seeking a high will continue to increase his or her dose to get past the "set point" the brain has established and get to the high.
Heroin, morphine, opium, and the synthetic opioids such as codeine, cause intense and painful withdrawal symptoms. About eight to twelve hours after the last heroin use, an addict's eyes water, he or she sneezes, and feels weak. Depression sets in. The addict experiences extreme muscle cramps and spasms (leg spasms may have led to the phrase "kick the habit.") He or she has chills and breaks out in goose bumps all over the body, which may be the origin of the phrase "quitting cold turkey." Other symptoms include nausea, vomiting, and diarrhea. Withdrawal symptoms become worse and worse over two to three days. Within a week to 10 days the illness is over. The good news about heroin withdrawal is that it is never fatal in individuals who are otherwise healthy. This means that if someone truly wants to quit the addiction, it is possible.