As with other diseases and disorders, the likelihood of developing addiction varies from person to person, and no single factor determines whether a person will become addicted to drugs. In general, the more risk factors a person has, the greater the likelihood that drug use will lead to drug use and addiction. Protective factors, on the other hand, reduce a person’s risk. Risk and protective factors can be either environmental or biological.
Biological factors that can influence a person’s risk of addiction include their genes, developmental stage, and even gender or ethnicity. Scientists estimate that genes, including the effects of environmental factors on a person’s gene expression, called epigenetics, account for between 40 and 60 percent of a person’s risk for addiction. Adolescents and people with mental disorders are also at greater risk of substance use and addiction than others.
National Institute on Drug Addiction: Drugs, Brains, and Behavior: The Science of Addiction