 For substance usedependence, marijuana MJ users are five times more likely than non-MJ users to smoke tobacco cigarettes, and regular caffeine users are twice as likely as non-caffeine users to smoke cigarettes. Cigarette smoking contributes greatly to morbidity and mortality among patients with drug and alcohol dependencies, making it vital to understand better the complex relationship between drugalcohol dependence and brain nicotine receptor densities in cigarette smokers. 