While such drug abuse is stabilising in many developed countries, it is increasing in East and South East Asia and the Middle East, the UNODC said.

A UNODC report said the annual use of synthetic drugs exceeded that of cocaine and heroin combined.

The global market for these types of drugs is estimated at $65bn (£37bn).

Many Asian countries have reported an increase in the use of methamphetamine.

And the report says there have been major seizures of amphetamines in Saudi Arabia.

Social changes

The head of the Vienna-based UNODC, Antonio Maria Costa, said the trend reflected social changes in these regions.

"It may be related to a large extent to the income dynamics in South East Asia and the Middle East," he said.

"We are referring to societies which are moving very rapidly; socially, economically, financially. No doubt it also refers to greater opening of markets.

"Border controls are inadequate in respect of what is needed to face the demand. So there is a supply effect and there is also a demand effect as well."

Mr Costa said that a decade ago, synthetic drugs were a cottage industry, but added that now they were big business, controlled by organised criminal groups.

Unlike plant-based drugs like cocaine and heroin, the production of synthetic drugs is hard to trace since the ingredients are readily available for legitimate industrial purposes.

BBC