This is an excerpt of an article I found while browsing..
Drug abuse is a brain disease that changes the brain physically and chemically. Addiction alters the way the brain's pleasure circuit (also called the reward circuit or hedonic circuit) works. Food, sex, gambling, personal power and novelty also increase the brain's dopamine (a brain neurotransmitter) level, create pleasure and may be addictive. But drugs change the brain over time, flood it with dopamine and eventually reduce the pleasure effect. Thus the addict takes more and more drugs just to feel less miserable. The pleasure is gone.
Cocaine stops the molecules that usually mop up excess dopamine. Amphetamines push the dopamine out of the sacs where it is stored. Heroin makes the dopamine-containing neurons fire more. Alcohol helps release more dopamine. Thus with an excess of dopamine, the addict feels high.
Drugs affect memory and even the recall of drug-situations can cause relapses. Starting is easy, stopping is not. Relapses are very common.
More can be found at
http://www.learnwell.org/addiction.htm