Officials in China have defended their decision to overturn a patent on the anti-impotence drug, Viagra.
Viagra is made by the world's biggest drug company, the American pharmaceutical giant Pfizer.
US business groups have condemned the decision, which they say has far-reaching implications.
But some Aids activists hope it could spell good news for Chinese people with HIV/Aids.
Sources at China's state intellectual property office said Pfizer had failed to provide enough information on exactly how the active ingredient - a substance called Sildenafil Sitrate - was manufactured.
It said several Chinese companies had filed complaints, and the authorities had now ruled in favour of allowing them to copy the drug.
Pfizer is reported to be appealing against the decision, arguing that it was not asked for the information when the patent was granted, three years ago.
US business lobby groups have condemned China's decision as making a mockery of its commitments under World Trade Organization rules to protect intellectual property.
On the other hand, one Aids activist said the move was potentially good news if it signalled a tougher Chinese line on the pharmaceuticals that might help to make foreign-patented anti-retroviral drugs available cheaply for Chinese people with HIV/Aids.
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