Drug use and drug addiction have been traditionally considered to be a male problem, however the gender gap has been decreasing over the past few decades. Thus, while the prevalence of alcohol, cannabis and nicotine dependence is still overall greater among men than among women, sex/gender differences in the abuse of stimulants and opiates seem to have disappeared. Moreover, women appear to be more prone to develop drug dependence, suffer more severe physical and psychological consequences of drug abuse, and have more difficulties quitting the habit. Numerous psychological, socio-cultural and biological factors have been implicated in these changing statistics. For example, while a large proportion of men initiate drug use to induce feelings of elation, energy or focus, women frequently start taking drugs to alleviate pre-existing mental health problems, including high levels of stress, feelings of alienation, depression, anxiety, or post-traumatic stress disorder. This maladaptive self-medication strategy often results in a faster transition to a habitual drug use and eventually a more severe dependence. In addition, the socio-cultural norms (particularly in the Western society) have changed dramatically over the past few decades. Thus, while there is still a more severe stigma and prejudice against women who use drugs (especially if they are pregnant of have children), overall there is much greater acceptance of women's drug use than it was several decades ago. Moreover, women have much greater access to various drugs of abuse than they used to have. Finally, over the past couple of decades new research started emerging pointing to some neurobiological factors that could also contribute to sex differences in drug addiction. Thus, there is now evidence that dopamine system, which for decades has been strongly implicated in drug reinforcement, is sexually dimorphic. The number of dopaminergic neurons, the density of the dopaminergic terminals, as well as responsiveness of the dopaminergic system to drugs of abuse, has been shown to differ between males and females and it has been shown to be modulated by sex steroid hormones, especially estrogen. For example, female rats exhibit greater motivation to self-administer cocaine than male rats and their motivation is the highest during elevated levels of estrogen. All these psychological, socio-cultural and biological factors that contribute to sex/gender difference in drug use and drug dependence, should be considered while evaluating and treating individuals with drug addiction problem.