Canadians are being warned by Health Ministry to be careful buying Tamiflu online and to avoid anything called "generic Tamiflu" because generic versions of the anti-viral drug do not exist.
Health Canada put out the warning Friday after reports from the United Kingdom that many websites -- including two Canadian ones -- have been selling illegal Tamiflu to people in the United Kingdom.
Health Canada said other reports also indicate the US Customs Border Patrol stopped more than 50 shipments of counterfeit Tamiflu at their border.
Jirina Vlk, media spokesperson for Health Canada, said there is no indication of counterfeit Tamiflu in Canada at present, but he warned that "it could be here."
The ministry advises Canadians to buy Tamiflu with a prescription from a doctor they know, at a pharmacy where they have a previous relationship.
People concerned about any Tamiflu they have bought should bring the drug to a pharmacy, their doctor or the drug manufacturer, Swiss firm Roche AG.
This latest concern about Tamiflu follows reports that the drug may not be a fail-safe defense against the avian flu, according to a New England Journal of Medicine report.
The authors of the report say they have found evidence the H5N1 virus can mutate into a form unaffected by Tamiflu, thus rendering the world's ever-growing stockpiles of the drug ineffective if the mutated strain were to spread.
Since 2004, the H5N1 virus has killed at least 71 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.
There is no proof yet that bird flu 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.
Tamiflu is known as the most effective weapon in the fight against avian or bird flu.