The most commonly injected drug is heroin, but amphetamines, buprenorphine, benzodiazepines, barbiturates, cocaine, and methamphetamine also are injected. Any water-soluble drug may be injected. Treatment of individuals who use injection drugs may be complicated by social and political barriers to treatment and by a lack of resources for public health approaches to treatment.

Both illegal drug production and injecting drug use have been globalized in recent years. Injecting drug use has diffused to countries that formerly had little problem with it. Both injecting drug use and HIV infection can spread rapidly within a community. Introduction of and rapid increase in injecting drug use is believed to be responsible for dramatic increases in HIV infection in some areas. In China, in Central Asia, and in several countries of Eastern Europe, injecting drug use is the primary risk factor for HIV infection.