By Madeleine Morris
BBC World Service Development Reporter
Anti-malaria drugs which save hundreds of thousands of lives every year are at risk of becoming useless, because of new counterfeiting tactics.
Malaria is one of the world's biggest killers
Experts have discovered counterfeiters in South East Asia are producing dilute fake versions of the drugs - raising the risk of resistance.
The drugs are based on compounds from the Chinese plant Artemesinin.
They are the only cheap drugs to which the most deadly malaria parasite has not developed resistance.
As such, they are a vital plank in the global fight against a disease which kills over a million people every year.
The fake versions being produced by counterfeiters do not contain enough of the active ingredient to kill the malaria parasite, falciparum.
So exposure to it gives the malaria the chance to develop resistance.
Paul Newton, a malaria expert from Oxford University, based in South East Asia, said: "This would be a disaster for malaria control globally.
"We may have malaria that could not be treated in any affordable way."
Dr François Nosten, of the Shoklo Malaria Research Unit on the Thai-Burma border, said artemesinin-based drugs were "irreplaceable".
"They are very cheap, and very, very effective and work anywhere in the world. It can mean life or death for millions of people."
A recent study found over half of all the drug Artesunate sold in Laos, Cambodia, Vietnam and Burma is fake.
It is believed the drug counterfeiters have begun adding small amounts of the active ingredient to their product to try and fool simple dye tests which test the veracity of artesunate.
These tests are widely used in South East Asia but they can only determine whether the active ingredient is present in the drug, not the amount.
It is not yet known how widely distributed the fake drugs with the resistance potential are, but given the size and sophistication of the counterfeiting business in the region, scientists are very concerned it could be widespread.
Should resistance to an artemesinin-based drug develop in South East Asia, it could spread to other parts of the world.
This would have very serious consequences for Africa, according to Dr Ambrose Talisuna, of the Ugandan Health Ministry and co-Secretary of the East Africa Network for Anti-Malarial Resistance.
"It would be a public health disaster. Malaria kills a very big proportion of people in Africa, predominantly children under five and pregnant women.
"So if we lose the most effective drugs to counterfeiters, people will die." The World Health Organisation recommends artemesinin in combination with other drugs as the most effective response to falciparum malaria, the most deadly strain.
It estimates 130 million doses of Artemesinin Combination Therapy (ACT) will be taken in Africa alone in 2006.
Dr Facundo Fernandez, of the Georgia Institute of Technology, first discovered the new counterfeiting technique two months ago when analysing batches of fake Artesunate.
He is appalled that counterfeiters are now threatening the most effective anti-malarial drugs for profit.
He said: "This is no different from plain murder.
"A person taking medicine to be cured of this type of disease is expecting science and medicine to help him.
"Instead he's getting nothing, and it's usually the most poor people on the planet."
Additional reporting by Orlando de Guzman for the BBC World Service US co-production, The World.