So-called new psychoactive substances being sold under harmless names such as "spice" or "incense" are causing the international drug control system to flounder, a new report from the United Nations has warned.

The substances pose a serious health risk although most of them are legal, the UN Office on Drugs and Crimes (UNODC) has said in its annual World Drug Report’, which was released on Wednesday.

"Sold openly, including via the internet, [new psychoactive substances (NPS)], which have not been tested for safety, can be far more dangerous than traditional drugs," it said.

From 166 known NPS's in 2009, the number has risen to 251 in 2012, according to the UN body. Levels of global drug use, however, appear to be generally stable, the report said.

These new drugs, which could be synthetic or plant-based and can easily be altered to create different varieties, are now developing at a rate that outpaces efforts to control or ban them, it said, adding that the long-term effects were unknown.

In the United States, NPS's were the most used drugs among students. In Europe, NPS use was also on the rise. NPS's were also present in Asia and Africa.

Worldwide, cannabis remains the most commonly used illegal drug, with 3.9 percent of the global population aged 15-64 using it, the report said.

In other findings, the agency reported that heroin and opium use remains steady at about 16.4 million people, or 0.4 percent of the world's adult population.

Prescription drugs misused

In 60 percent of countries, however, prescription drugs such as sedatives and tranquilisers were among the top three most misused substances, which was "of particular concern", the report added.

The market for amphetamine-type stimulants (ATS), which include ecstasy and methamphetamine, was also growing, especially in east and southeast Asia, the UNODC found. Ecstasy use was reported to be on the rise in Europe.

Meanwhile, eastern and western Africa are becoming increasingly important areas for drug traffickers.

A new maritime route seemed to have developed from Afghanistan through Iran and Pakistan leading south towards Africa, "a worrying trend" given the lack of trafficking information on that continent, the UNODC said.

"Africa is increasingly becoming vulnerable to the drug trade and organised crime," UNODC chief Yuri Fedotov said.

War-torn Afghanistan produced 74 percent of the world's opium in 2012, according to the UNODC.

Source: Agencies