Observations of drug-related cues may induce craving in drug-dependent patients,prompting compulsive drug-seeking behaviour.Previous studies have examined brain activation for drug reward and natural reward (such as sex and food).This has not been done in relation to ketamine use,which poses an increasingly widespread problem.Here,we determined brain responses to ketamine,smoking and erotic cues in chronic ketamine users with smokers and non-smokers as control groups.