The rapid emergence of synthetic new drugs, often sold online as \"legal highs,\" represents a significant challenge for policy makers in the coming decade, a European Union drugs agency said Tuesday. The European Monitoring Centre for Drugs and Drug Addiction said in its annual report that while cocaine use across Europe may have peaked, and that ecstasy and amphetamine use is stable or declining, new synthetic substances are continually being developed. So far this year, 39 new substances have been reported to the European early-warning mechanism, with another 41 identified in 2010. \"Europe’s drug policies and responses must now be configured to face the challenges of the next decade,\" agency director Wolfgang Goetz said in the report. The biggest difficulty for authorities is the large number of online stores offering \"legal highs,\" including a record 600 shops touting psychoactive substances. One such drug, the synthetic stimulant mephedrone, is frequently sold online and through illegal networks, the report states. \"The fast-moving and increasingly joined-up world we live in is mirrored by an increasingly fast-moving and joined-up drug market which appears quick to adapt to both threats and opportunities,\" Goetz said. The report is more optimistic with cocaine trends. It states the drug\'s high street price and the financial crisis have prompted a cutback in cocaine use among young adults in several countries including Britain, Denmark, Spain and Italy -- a trend that\'s also been observed in Canada and the Unites States. Cannabis remains Europe\'s most popular illicit drug in Europe, though its use is decreasing among youths. Amphetamine and ecstasy use is also stable or in decline, the report states.