VIENNA: Designer drugs are multiplying at a worrying rate and increasingly sending users to hospital, a UN-affiliated report said Tuesday, calling for international efforts to stem the spread of these substances.

"In recent years, there has been an unprecedented surge in the abuse of new psychoactive substances," often called "legal highs" or "designer drugs," the Vienna-based International Narcotics Control Board (INCB) said in its annual report.

"In Europe alone almost one new substance is appearing every week. Previously, between 2000 and 2005 there were an average of five notifications of new substances per year," added INCB head Raymond Yans.

Experts estimate thousands of such drugs currently exist on the market, the report said.

"As abuse of these substances has increased, so too has the number of users who have experienced grave health consequences or even suffered death due to exposure to them," it went on, pointing to "marked increases in emergency room visits for adverse health reactions" and "significant increases in calls to poison treatment centres."

Aggravating the problem further was the fact that these drugs were not subject to international control regimes and were readily available on the Internet, requiring states and international institutions to find new ways to identify the drugs and stop their distribution, the UN-affiliated INCB said.

Another increasing problem was the rise in prescription drug abuse, including painkillers, sedatives and others used to treat epilepsy, attention-deficit hyperactivity disorder and mental illnesses.

This was especially the case in North America, Southeast Asia and South Asia, while prescription-drug abuse via injection was especially prevalent in South Asia, heightening the risks of HIV and hepatitis B and C.

In Latin America, "more than six percent of secondary school students have already abused tranquillizers in some countries," the INCB found.

It also condemned the easy availability of drugs and poor prescription policies in North America.

The United States, most particularly California, meanwhile came under scrutiny for its medical cannabis scheme, which according to the INCB "has contributed to an increase in cannabis abuse."

The report called on Washington to better police such programmes to remain in line with international conventions and ensure that the drugs do not become available for recreational use.

The Board makes annual recommendations on narcotics policy to states but also international institutions, including the United Nations.