Though less common in the general population, illicit addictions such as cocaine and opiate dependence can be more devastating socially, cause more physical illnesses, and represent an extreme of addiction. Because illicit drug addiction is less common, large scale GWAS have not been undertaken as yet. Instead, the approach to studying the genetics of drug addiction has been through candidate genes. Hundreds of candidate gene association studies have been performed in the pre-GWAS era for drug addiction as well as for nicotine and alcohol dependence. Thousands more candidate gene studies have been undertaken for other medical illnesses. However, a disconnect between these reported candidate gene associations and the findings from GWAS studies exist in both the addiction field and across all of medicine. If these candidate gene studies are valid, then the GWAS studies should identify thousands of genetic variants that play a role in disease. Though hundreds of genetic variants have been conclusively confirmed by GWAS as contributors to complex diseases, the number of confirmed genetic variants is more modest than what is expected from the candidate gene studies.
