Global opium production fell by 38% in 2010 and cocaine cultivation continued to decline, according to the annual UN report on the world drug market.

The report said that while the global markets for cocaine, heroin and cannabis had declined or remained stable, the production and illicit use of prescription opioid drugs and new synthetics known as legal highs, which mimic the effects of traditional drugs, had increased sharply.

The UN estimates that 210 million people, about 5% of the world's population, used some kind of illicit drug last year at least once. The most popular drug remains cannabis, with 170 million users. An estimated 39 million "problem drug users" use heroin, cocaine and other class A substances.

Sandeep Chawla, director of policy of the UN office of drugs and crime, said the rise of new synthetic drugs reflected their lack of dependence on plant cultivation. Instead they could be produced from readily available industrial chemicals close to potential consumers without the need to set up global trafficking chains.

The sharp decline in opium production to 4,860 tonnes was due to a blight that wiped out most of the opium harvest in Afghanistan last year, the report said, although experts expect it to recover this year. A 20% increase in opium production in Burma did little to compensate.

The UN said it was more encouraged by the continuing decline in the area under coca cultivation, which has shrunk by 18% since 2007 to 149,000 hectares.

The last decade has seen coca cultivation in Colombia more than halve from 163, 300 hectares in 2000 to 62,000 last year. The UN experts say this decline has not been offset by small increases to 61,200 hectares in Peru, which on one measure has replaced Colombia as the largest producer of coca in the world, and in Bolivia.

The UN's policy director also pointed to successes in containing the emergence of West Africa as a major transhipment point into Europe for cocaine over the past decade. It is estimated that about 21 tonnes of cocaine were trafficked via West Africa to Europe in 2009 – down from 47 tonnes two years earlier.

Chawla also cited the fact that the majority of seizures now took place in South America rather than US or western Europe as further evidence of progress.

The UN report describes the fall in Colombian coca cultivation, which declined a further 15% last year, as remarkable but sounds a cautious note about the actual impact on production. Chawla said increased yields and changes in the way the leaves are processed meant the jury was still out on whether the decline was reflected in falling cocaine production levels.

However, Chawla did say that increased counter-narcotics operations, including fumigation and eradication programmes, and the withdrawal of Farc rebels from parts of the country, contributed to the sharp decline in cultivation.

It has taken a decade of US support and more than $5bn (£3bn) in aid through Plan Colombia. In 2009 and 2010, Colombian authorities seized at least 10 times more cocaine than their Peruvian counterparts – and half of it was caught before it even crossed the border.

Coca's traditional home in Peru is in the central jungle valleys on the eastern slopes of Andes cordillera. Around half of Peru's cocaine comes from one in particular, the Ene-Apurimac river valley.

Conditions are perfect for growing coffee and coca but most of the 350,000 population – nearly half of whom live in dire poverty – choose to grow coca. For many families it is their caja chica – a "small box" from which they can get ready cash for school uniforms, extra supplies or just keep something for a rainy day.