A drug stock-out is defined as an event that a drug-outlet or group of outlets of any type (drug shop/pharmacy/clinic), which is licensed or unlicensed, which serve(s) a specific community, are out of stock of a drug that is in demand in that community.

Although drug stock-outs are very common, their magnitude is largely undocumented in global health because of a lack of transparent demand data and inventory recording mechanisms. Drug stock-outs can increase the number of uncomplicated infections turning resistant, prescription of inappropriate drug alternatives, prolonged or severe illnesses due to inability to continue drug regimen, frustration of health care workers, and even death.