Novice begins and continues a potentially compulsive activity because of pleasurable sensations. The body adapts, tolerance.
Continuation alleviates despair and discomfort.
After stopping the use of the drug, enzyme levels slowly change, but there is a limitation to the body's ability to 'revive' old levels of activity. Conscious use of the drug to avoid or suppress the withdrawal symptoms. Not all heroin users are physically addicted. Non addicting drugs are used in the same way as classically addicting drugs. Cocaine and animal studies.