Drug contain chemicals that tap into brain's communication system and disrupt the way nerve cells normally send receive and process information there are at least two ways that drugs cause this disruption (1) by imitating the brain's natural chemical messengers and (2) by overstimulating the reward circuit of the brain.
Some drugs have a similar structure to chemical messenger called neurotransmitter which are naturally produced by the brain this similarity allows the drugs to fool the brain's receptors and activate nerve cells to send abnormal messages.
Other drugs such as cocaine or methamphetamine can cause the nerve cell to release abnormally large amount of natural neurotransmitter or to prevent the normal recycling of these brain chemical which is needed to shut off the signalling between neurons the result is a brain awash in dopamine a neurotransmitter present in brain region that control movement emotion motivation and feelings of pleasure the natural behaviors linked to survival producing euphoric effects in responce to psychoative drugs this reaction sets in motion a reforcing abusing drugs.
As a person continues to abuse drugs the brain adapts to the overwhelming surges in dopamine by producing less dopamine or by reducing the number of dopamine receptor reward circuit the result is a lessening of dopamine's impact on the reward circuit which reduce the abuser's ability to enjoy not only drugs but also other events in life that previously brought pleasure this decrease compels the addicted person to keep abusing drugs in an attempt to bring the dopamine function back to normal but now large amount of the drug are required to achieve the same dopamine high an effect know as tolerance.
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