People who have been building up a personal stockpile of the flu drug Tamiflu should stop, or they risk becoming a "walking mutated virus bomb," warns a Toronto infectious disease expert.

"Before you didn't really have this risk to others. Now you've got this risk to others by stockpiling this drug," Dr. Neil Rau told CTV's Canada AM on Thursday.

Rau made the comments after a study was released Wednesday warning that the H5N1 virus can mutate into a form unaffected by Tamiflu.

According to the study, completed by Dr. Menno de Jong at the Hospital for Tropical Diseases in Ho Chi Minh City, Vietnam, four out of eight avian flu patients who were given the medication died despite the treatment.

In one case, a 13-year-old girl suffering from H5N1 received Tamiflu treatment a day after she developed symptoms -- the optimal drug window is within 48 hours of symptom onset. Yet she developed resistance to oseltamivir.

"I think my philosophy on the stockpiling has been changed significantly by this new development," said Rau.

For one thing, he said that bird flu may not become the strain that causes a pandemic. For another, taking Tamiflu in a way not recommended by doctors could create a risk to others.

"Because you might take this drug in a very chaotic fashion and become a sort of a walking mutated virus bomb that you could then spread this problem to other people," said Rau.

"So in the interest of defending yourself, you are actually posing a potential risk to other people."

This was also the message in another article published in the New England Journal of Medicine -- the same journal that published the Tamiflu study.

Drs. Allan Brett and Abigail Zuger, argued that taking Tamiflu could lead to a rise in resistance if people take the drug inappropriately, and people could take it when they don't even have the flu. As well, personal stockpiling exhausts limited supplies of the drug meant for the treatment of people with human flu.

The World Health Organization said Thursday that signs that the H5N1 bird flu virus may be developing resistance to Tamiflu in some patients is not necessarily cause for alarm.

"It just points out the need for more information... What really is critical is understanding whether the way we are using the drugs contributes to that (resistance)," Keiji Fukuda, an expert at the WHO's global influenza program, told Reuters.

Tamiflu was considered the No. 1 weapon in the pharmaceutical arsenal against pandemic flu. Now flu experts are debating whether the current dozing regime is adequate to combat the infection, and a study has been set up in Southeast Asia on the issue.

Most experts, including Rau, agree the Tamiflu is the best defence against the common flu strains.

Fukuda said they will have to continue monitoring the situation where H5N1 is concerned. He said that using doses that are too small, or for not long enough, can contribute to resistance developing.

Bird flu has not yet appeared in North America and there is no proof that it can spread from person to person. But officials worry that if the virus mutates, it could become as contagious as the annual flu, but much more deadly.

Since 2004, the H5N1 virus has killed at least 73 people in Asia. According to figures updated by the World Health Organization on Dec. 16, there have been at least 139 human cases, including 95 this year alone.

More than 200 companies and governments have asked the drug's maker, Swiss firm Roche AG, if they can help manufacture Tamiflu. So far, Roche has allowed Indonesia, Thailand and the Philippines to produce the drug without paying compensation. Tamiflu is not patent protected in those countries.