The unprecedented growth in "legal highs" has led to 348 new types of synthetic drugs appearing in more than 90 countries in every region of the world, the UN drug agency has reported.

The UN office on drugs and crime said the majority of the new psychoactive substances had emerged only in the past five years and the actual number available may be significantly higher than the 348 officially reported so far.

Nearly 100 new substances have emerged in the last year alone, in further evidence of the establishment of a new global market.

The UN special report said the fact that legal highs were no longer restricted to niche markets but available worldwide was a particularly worrying development.

The agency said the evidence from almost all regions was that synthetic drugs were gaining popularity among young people.

In parts of Central and South America, the use of amphetamine-type substances was more popular than cannabis or cocaine among some age groups. In North America and Europe, certain legal highs were more widely used than traditional illicit drugs among younger age groups.

The only figures so far published for the use of legal highs in Europe date from 2011. The global market is growing so rapidly that they are likely to be far higher now. They showed that 4.8% of 15- to 24-year-olds – 2.9 million people – had experimented with new substances that imitate the effects of illicit drugs. The highest rates were reported in Ireland, at 16.3%, Poland at 9%, Latvia at 8.8% and the UK at 8.2%.

The UN said the global market for legal highs was very dynamic and a number of the new substances were transient. "Therefore, the fact that certain new psychoactive substances are present does not mean that they have an established market."

The UN experts said that so far none of the 348 new substances reported globally in 94 countries by the end of 2013 were under international control.

"However, in 2014, the United Kingdom requested the international control of mephedrone, a potentially fatal recreational substance, under the 1971 UN convention on psychotropic substances," they added.

The special report says that data on drug use shows that the British ban on mephedrone, which was marketed as "meow meow", and its classification as a class B substance, did lead to a small decline in its use – from 1.4% to 1.1% of people aged 16 to 59 – but it still remained popular in London clubs.

The UN report showed that designer drugs that imitated the effects of cannabis were the most popular with the number of synthetic cannabinoids, as they are called, soaring from 60 in mid-2012 to 110 in 2013.

They also reported evidence from almost all regions of the world that tablets sold as ecstasy or methamphetamine contained not just the touted ingredients; they also increasingly comprised chemical cocktails that posed unforeseen public health challenges.

"Emergency services may therefore find themselves unable to identify life-threatening substances and powerless to administer the proper treatment to users," the UN report added.