The UN drug-monitoring agency says the abuse of prescription drugs has become a major problem in parts of the world.
The agency says that in some cases it rivals the use of heroin and cocaine.
The International Narcotics Control Board says the non-medical use of prescription drugs, such as painkillers, is particularly alarming in the US, Canada and Mexico.
The agency has warned that the increased use of illegal Internet-based pharmacies could result in the spread of the trend to other countries.
