Drug addiction has four components:

-Tolerance: On  repetitive administration a drug  becomes progressively less  effectual, so the  dose  has  to be  increased if the  real action is to be  maintained. The precise method for tolerance depends on the drug but involves enzyme induction, modify in receptor numbers and alterations to second messengers. Tolerance does not necessarily lead to addiction; for instance, moderate drinkers develop tolerance for ethanol without becoming alcoholics.

-Dependence: the dependence is occurs when biological changes brought about through the drug are such which normal functioning is only possible when the drug is present.

-Abstinence (withdrawal) syndrome: If a drug is withheld after dependence is established an abstinence syndrome results which is extremely unpleasant and lasts until the long-term biological changes that brought about dependence have abated. Hence, addiction (the need to take the drug repeatedly) can be driven as much by the aversion to withdrawal as by the positive reinforcing qualities of the drug. In rare cases with some drugs (e.g., alcohol) withdrawal can be fatal.

-Craving: The intense longing for a drug felt by addicts is a learned response that long outlasts the abstinence syndrome. Addicts  form  memories that associate the  pleasure  produced through the  drug  with  the  environment and  cues  which accompany the  drug taking. Subsequent exposure to the similar context causes craving. Brain imaging shows that cocaine-addicted subjects have increased activity in the cingulate and orbitofrontal cortices in response to stimuli associated with cocaine availability but decreased activity when presented with stimuli associated with natural rewards, compared to non-addicts. This corresponds with the much more intense motivation of addicts to seek the drug than natural rewards. Craving is the major barrier to the successful permanent rehabilitation of addicts.