Fake prescription medicines are swamping developing nations with sometimes deadly consequences, a report by the UN drugs watchdog has said.
Prescription drug abuse will soon overtake illegal drugs, the UN says
The International Narcotics Control Board report says up to 50% of the medicines in these markets are fake.
It also says abuse of prescription drugs will soon overtake that of illegal narcotics worldwide.
The board said governments had to enforce existing laws and bring in new ones on illegal internet drug sales.
Courier services, the general mail and local markets were other major means of counterfeit sales, it said.
The report said the fakes market was "increasing rapidly".
"[It] exposes patients to serious health risks by providing access to poorly or incorrectly labelled medicines that are ineffective, substandard and, in some cases, even lethal," it said.
Couriers, the internet and local markets trade fake drugs, the UN says
The board's president, Philip Emafo, said: "Gains over the past years in international drug control may be seriously undermined by this ominous development if it remains unchecked.
"Instead of healing, [fake drugs] can take lives."
The report said the UN and the World Health Organisation should help member states that do not have the resources to tackle the counterfeiters and traffickers.
The Vienna-based drugs watchdog also said prescription drug abuse had outstripped heroin, cocaine and Ecstasy in some parts of Europe, Africa and South Asia.
Only cannabis was more abused than prescription drugs in the US, it said.
Mr Emafo said exact figures on prescription abuse were hard to find as many countries were unaware which drugs were being abused.
He said: "The very high potency of some of the synthetic narcotic drugs available as prescription drugs presents a higher overdose risk than the abuse of illicit drugs."
The BBC's Jill McGivering says unregulated internet sales are causing growing fears in the developed world.
A recent study found that almost 90% of internet pharmacies did not require a prescription to send out controlled drugs, she says.
The report also said Iran was now the world's top abuser of opiates, mainly because of its close proximity to sources in Afghanistan.