A record number of new drug compounds were detected in Europe last year, according to the EU's monitoring body.
Its joint report with European police forces said synthetic drugs were emerging at an "unprecedented speed", driven by an online marketplace.
These "legal highs" were a growing challenge to drugs control in the EU.
The European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) found 24 new psychoactive substances through its international early warning system.
In its joint annual report, the EMCDDA and Europol said most of the substances were synthetic cannabinoids or various types of cathinones which are related to amphetamines.
Synthetic cannabis, typically sold under the name Spice, was banned in the UK in December 2009.
The most well known cathinone in the UK, Mephedrone, was banned just before the general election was called, amid an ongoing row between scientists and the government over the lack of clear evidence on its effects.
Synthetic drugs are designed in laboratories to replicate the effects of other already banned substances.
In most cases, the new drug's composition is almost identical to a banned substance, except for slight chemical modifications to the compound.
The Lisbon-based drugs body said the 24 new substances identified in 2009 were almost double the previous record of 13 new compounds found just a year before that.
The report said that 2009 saw nine new synthetic cannabinoids detected in the drugs market from four distinct chemical groups.. There were four synthetic cathinones.
With so many synthetic drugs now emerging, the monitoring body said it was becoming harder for law enforcement agencies and current laws to keep up.
"The appearance of a large number of new unregulated synthetic compounds marketed on the Internet as 'legal highs' or 'not for human consumption' and specifically designed to circumvent drug controls presents a growing challenge to current approaches to monitoring, responding to and controlling the use of new psychoactive substances," said the report.
"It can be anticipated that the concept of 'designer drugs' will continue to change at an unprecedented speed.
"With rapid technological developments, for example cheap organic synthesis coupled with the increased use of the Internet for marketing and selling new of drugs, it may be expected that synthetic analogues of other major drug groups will appear.
"New synthetic opioids and cocaine derivatives have already been identified via the Early Warning System, albeit as isolated cases."
It added that there had also been an increase in the presence of piperazine on the drugs market which appeared to have replaced the more common compound of MDMA in ecstasy.