By Richard Black BBC science correspondent

Researchers in Chile say there is a growing problem of people buying and selling Aids drugs on the black market.

They say people are trading their own medicines to buy food, while others are buying drugs of dubious quality.

Some people may be taking one drug one time, and the next week they'll be buying another drug

Dr Christian Morales, researcher

The researchers say the problem poses a major public health risk.

The results of their study have been presented to the United Nations Aids conference in Barcelona, where thousands of delegates are discussing the fight against the disease.

Problem worsening

The researchers from Chile and Canada do not know how much the black market is worth, but say it has expanded since 1999 when they started investigating it.

They suspect that between 10-15% of people taking anti-retrovirals in Chile are trading on the black market.

	:	Aids in South America
Click to see South America's growing problem

According to Dr Christian Morales from the University of Montreal, who presented the findings at the conference, some of the drugs circulating are of unproven quality, and supplies are unreliable.

"A lot shortages and supplies can arrive... so some people may be taking one drug one time, and the next week they'll be buying another drug," she said.

Dr Morales believes the black market has grown up for several reasons: some people are buying cheap Aids drugs outside Chile and selling them inside the country for a profit.

Poorer Chileans who receive free drugs from the government health system are selling them in times of hardship for food and other essentials.

Cover up

By interrupting their treatment, HIV sufferers risk accelerating the progress of their illness, and they are also helping to create drug resistant strains of the virus.

And some people with private health insurance choose to buy on the black market, rather than having to tell their insurance companies they are HIV positive.

The researchers are urging the government to expand its free drug program to cover everyone with HIV, which they say would get rid of the problem.

And they say monitoring should be stepped up to try and gauge exactly how big the black market is.