Many studies have been conducted about drug addiction and the effects it has on a user. How the Brain responds to natural rewards and drugs.
The National Institute on Drug Abuse (NIDA) provides many sources of research on the topic and science of drug abuse and addiction.
NIDA also provides a list of Research Studies.
Scripps Research Institution, a “a vital segment of medical research that seeks to comprehend the most fundamental processes of life,” also shares numerous sources of research explaining the science behind addiction.