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Research has revealed some major differences between the brains of
addicted and nonaddicted individuals and to indicate some common
elements of addiction, regardless of the substance used. Science has
also provided compelling evidence that the development and
manifestation of addiction is influenced by genetic, biological,
psychosocial and environmental factors. And, the behaviors of drug
addiction are often characterized by an impaired control over drug
craving and compulsive continued drug use despite harmful
consequences.

Most addictive drugs create the sensations they do because they
imitate the brain’s natural chemicals, called neurotransmitters. Brain
cells transmit messages throughout the nervous system when they
receive enough of a given neurotransmitter. Addiction to opioid drugs
is particularly challenging because the brain produces natural opioid
substances and does not discriminate between them and opioid drugs
taken externally. The brain’s own natural opioids are extremely
important for survival and are involved in a variety of behaviors
including analgesia, mood, digestion, blood pressure, body
temperature, respiration and sleep. They also seem to play an
important role within the brain’s pleasure/reward system.
Interestingly, the brain responds to external opioid drugs, including
heroin, as if they are also biologically essential for survival.

When the addiction is established, the brain’s own chemistry becomes
imbalanced and the nervous system begins to rely on the external
opioid to try and regain balance. The problem is, when the external
opioid drug is removed by metabolism, the nervous system is left in an
unbalanced state resulting in withdrawal pain and craving for more of
the drug. The addicted person in this state becomes physically,
emotionally and mentally dysfunctional unless and until more opioid
drug is taken.

The original methadone research suspected that in many cases, the
person addicted to heroin has some kind of physiologic disorder,
possibly with an underlying genetic predisposition that manifested
itself as compulsive drug seeking/using behaviors after an initial
exposure to certain types of drugs. Research then speculated that