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According to the article Drug Facts: Understanding Drug Abuse and Addiction and Drugs, Brains, and Behavior: The Science of Addiction, addiction is a disease caused by some types of drugs. Specifically, they rewire the brain to cause an overstimulus of the 'reward' centres and by imitating some of the pre-existing neurochemical messengers in the brain - essentially, 'tricking' the brain that homeostasis is maintained.
Related, the article "Why does the rapid delivery of drugs to
the brain promote addiction?" (Samaha and Robinson, 2005) concluded that the rate of delivery is an important factor:
that rapidly
delivered drugs might promote addiction by promoting
forms of neurobehavioural plasticity that contribute to
the compulsive pursuit of drugs.