Studies have shown that 40-60 percent of the predisposition to addiction can be attributed to genetics.
This is not just the risk conveyed by genes themselves, but also the added impact of the environment on how those genes function or are expressed. Taking drugs is, of course, a necessary environmental risk factor for addiction, but so are exposures to stress and peers who use drugs. Growing knowledge about the dynamic interactions of genes with the environment and developmental stage confirms addiction as a complex and chronic disease of the brain with many contributors to its expression in individuals.
As a result of scientific research, we know that addiction is a disease that affects both brain and behavior.