Humans have always used drugs, probably as part of their evolutionary and nutritional heritage. However, this previous biological adaptation is unlikely to be so in the modern world, in which 2 billion adults (48% of the adult population) are current users of alcohol, 1.1 billion adults (29% of the adult population) are current smokers of cigarettes and 185 million adults (4.5% of the adult population) are current users of illicit drugs. The use of drugs is determined largely by market forces, with increases in affordability and availability increasing use. People with socio-economic deprivation, however measured, are at increased risk of harmful drug use, as are those with a disadvantaged family environment, and those who live in a community with higher levels of substance use. Substance use is on the increase in low-income countries which, in the coming decades, will bear a disproportionate burden of substance-related disability and premature death.