Swiss pharmaceutical group Roche has taken a knock from new evidence that bird flu is developing resistance to the antiviral drug Tamiflu.
Countries are stockpiling the Tamiflu drug to battle bird flu
A New England Journal of Medicine (NEJM) report warned the drug may become ineffective as the virus could develop more quickly than expected.
Two patients in Vietnam died of the virus despite treatment with Tamiflu, the report added.
Roche shares slipped 0.68% in midday trade following the news.
But Roche argued the current H5N1 strain of bird flu would have to mutate in order to transfer from human to human, rather than just from birds to humans as it does at the moment.
Experts have voiced concern that if it does mutate into a strain that passes between humans, it could spark a pandemic that would kill millions.
But World Health Organization (WHO) experts played down any fears concerning the drug, saying signs that the virus was developing resistance should not give cause for alarm.
"Whenever you use any kind of drugs, antivirals or antibiotics, you expect to see resistance develop," Keiji Fukuda, a scientist at the WHO's global influenza programme, told Reuters news agency.
Countries are currently stockpiling Tamiflu as a precaution in the event of a pandemic.
Roche also said it was looking at ways of using the drug should such a crisis occur - with one possibility being to increase the dosage of Tamiflu.
"This could quickly shrink the calculated coverage of the current stockpile and hence increase demand for pandemic stockpiling further," Bank Leu analysts said in a research note.
Meanwhile, US authorities have now given their backing to the use of Tamiflu as an influenza treatment for children aged between one and 12.