New drugs that evade legal bans are being developed at such a rate that they now outnumber illegal substances, the UN Office on Drugs and Crime (UNODC) said Wednesday in Vienna.
Such "legal highs" mimic the effects of existing drugs like amphetamine, cannabis or khat but can be far more dangerous, as little is known about their side effects and their potential to cause addiction, the UNODC said in its annual report.
Countries had reported 251 new substances to the Vienna-based UN agency by mid-2012, more than the 234 drugs that are banned under international treaties.
"The international drug control system is floundering, for the first time, under the speed and creativity of the phenomenon known as new psychoactive substances," the UNODC said.
Governments around the world have found it hard to outlaw these so-called designer drugs, because their chemical formulas are different from traditional substances.
Most of the new substances are made in Asia and Europe.
In the European Union, nearly one in every 20 young people has tried new drugs that are often sold as herbs or bath salts, the report said.
Meanwhile, use of traditional drugs such as heroin, cocaine and amphetamine remained stable, even though the UNDOC highlighted some worrying trends.
As opium production in South-East Asia is not keeping pace with Asian demand for heroin in countries like China, more Afghan opium is making its way eastwards, the report said.
Afghanistan remained the world's largest opium producer last year and poppy was grown on more and more fields. However, output fell due to a plant disease.
New data showed that the global number of people who inject drugs and are infected with HIV was only 1.6 million in 2011, 46 per cent below previous estimates.
Cultivation of coca remained stable in South America in 2011, according to the latest available data.
At the same time, the UNODC said cocaine was also shifting from Western countries to Asian markets such as Hong Kong and China, where it is seen as a lifestyle drug, and to Central America, where it is used along trafficking routes.