How specific are the genetic risk factors for SUDs? Studies that address this question have tended to discriminate between illicit and licit psychoactive substances. Illicit substances include drugs that are illegal (for example, cocaine or hallucinogens) and legal prescription drugs (for example, codeine) used in ways other than medically indicated. Western societies have generally deemed three psychoactive drugs to be ‘licit’ and appropriate for use by its adult citizens: caffeine, nicotine and ethanol. Although the specific symptoms of intoxication, tolerance and withdrawal differ substantially across specific psychoactive substances, and many physiological effects differ substantially across drugs, there is no clear physiological division between the effects of licit versus illicit substances.
