Governments everywhere are struggling to cope with an increase in the number of new drugs known as "legal highs", according to a UN report.
The United Nations Office on Drugs and Crime (UNODC) says the use of traditional drugs such as heroin and cocaine is globally stable.
But new synthetic substances are being constantly spread via the internet, the office's latest World Drug Report says.
It stresses that these seemingly legal drugs can have deadly effects.
These "new psychoactive substances" (NPS) have not been tested for safety and pose "unforeseen public health challenges", the report notes.
"Sold openly, including via the internet, NPS... can be far more dangerous than traditional drugs.
"Street names, such as spice, meow meow and bath salts mislead young people into believing that they are indulging in low-risk fun," the report adds.
New substances are being identified all the time and the authorities are struggling to keep up, according to the UNODC.
"While law enforcement lags behind, criminals have been quick to tap into this lucrative market," the report says.
It focuses on drugs that appear to originate in Asia but are marketed globally online.
The biggest market is the US, where use of these substances among youth "appears to be more than twice as widespread as in the European Union", it says.
Within the EU, Britain is a particularly receptive market, the UNODC says, with almost 700,000 Britons aged between 15 and 24 having experimented with legal highs.
The European Monitoring Centre for Drugs and Drug Addiction reported earlier this year that it had detected 73 new substances last year, compared with 49 in 2011.