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"Genetics load the gun, and environment pulls the trigger.”
Research is showing that people with a certain gene have a tendency towards being unable to quit once they have started drinking or using drugs. However, if an individual has a great support system of family and friends, a good education and the understanding of a family history where addiction has been threading its way through the genrations, then addiction is certainly preventable.
For more on addiction and genes, here is a video worth watching.