There are currently only four flu drugs marketed to battle flu: amantadine and rimantadine, which are called adamantane drugs; and oseltamivir (Tamiflu) and zanamivir (Relenza), which are neuraminidase inhibitors. Rimantadine is not sold in Canada.

In places such as China, drug resistance to H5N1exceeded 70 per cent to adamantane drugs, suggesting that they will probably no longer be effective for treatment or as a preventive in a pandemic outbreak of flu.

Chui said that manufacturers should develop an injectible version of Relenza, since "high doses can be given where necessary and onset time is a lot faster."

Intravenous Relenza would also ensure faster onset, which would be critical in patients who are seriously ill.

"We don't have to worry about absorption, injections take drugs right in. But if the patient takes them orally, maybe some amounts won't be absorbed or some may be destroyed by stomach acids," said pharmacist Raymond Mak at Queen Mary Hospital.

"Orally taken drugs take three to four hours to reach maximum blood concentration and three to four hours is very critical in severe cases. But injectable Relenza takes only 30 minutes to reach maximum blood concentration, this is a huge difference," Chui said.

While the H5N1 virus is now mostly passed directly from bird to human, health experts have warned that it is just a matter of time before it mutates into a form that is easily transmissible between people.