The UN 's International Narcotics Control Board (INCB) calls for stricter enforcement of control measures and public education campaigns about the risks from the misuse of prescription drugs, including painkillers and tranquillisers.

Their abuse has already outstripped that of heroin and cocaine in some parts of the world, says its report.

President of the INCB, Dr Philip Emafo explained that appetite suppressant drugs, also known as anorectics, have a use in the treatment of life-threatening obesity when prescribed and monitored by doctors.

"However, they are instead being used indiscriminately to feed the slimming obsession that affects some societies."

Growing trade

The scale of the problem is not clear.

Last year, a study found more than half of 1,230 UK women surveyed by Closerdiets.com admitted using slimming pills.

There are 14 different appetite suppressants that have been developed to treat obesity and other conditions, including attention deficit hyperactivity disorder and narcolepsy.

Professor Hamad Ghodse, former president of the INCB and chairman of addiction psychiatry at London's St George's Hospital, said: "We need to know the size of the problem in the UK.

"We think probably in the UK there are adequate regulations in place, but there needs to be the implementation of these regulations."

Dr Emafo said: "It is important for consumers to realise that what they think is a cut-price medication bought on an unregulated market may however have potentially lethal effects whenever the consumed drugs are not the genuine product or are taken without medical advice.

"Instead of healing, they can take lives."

The Medicines and Healthcare products Regulatory Agency said the global availability of counterfeit medicine had increased in recent years, through unregulated web sites operating on the internet.

A spokesman said that in the past 12 month, the MHRA had taken action against 30 web sites illegally supplying medicines, including counterfeits.