The European Monitoring Centre for Drugs and Drug Addiction (EMCDDA) said some 50 new substances have already been detected this year.
A total of 49 new "psychoactive" substances were officially notified for the first time in 2011, the EMCDDA said, up from 41 in 2010, and 24 in 2009.
And preliminary data for 2012 showed no signs of a decline, with more than 50 already detected this year.
The agency said Europe faced an increasingly complex stimulant market.
Cecilia Malmstrom, European Commissioner with responsibility for the EMCDDA, said: "Stimulant and synthetic drugs play a central role in the European drug situation, creating a market which is fast-moving, volatile and difficult to control. Data from emergency rooms, toxicology reports and drug treatment centres indicate that the associated risks are not always well known by the users."