Roche has issued a guide to consumers advising how to spot fake copies of its influenza drug Tamiflu, which is fast becoming a target for counterfeiters.

The trade in illegal copies of Tamiflu (oseltamivir) is being driven by consumers’ concerns about bird flu, and the fear that the H5N1 strain infecting birds around the globe may mutate and cause a pandemic in humans.

During the past two years, the powerful flu has killed millions of birds and more than 70 people in China and Southeast Asia, and cases of infected birds have been creeping into Europe, Africa and the Middle East. Countries around the world are busily stockpiling Tamiflu, a similar drug from GlaxoSmithKline called Relenza (zanamivir) and experimental flu vaccines in order to prepare for a pandemic, which some experts have estimated could kill millions of people.

But with Tamiflu in short supply until very recently as Roche struggled to increase its manufacturing capacity, people have been trying to get their hands on personal supplies of the drug to protect themselves and their families. Last October, when demand for the drug was outstripping supply, US Health and Human Services said the situation was ‘ripe’ for counterfeiting.

And sure enough, in December, US customs seized shipments of counterfeit Tamiflu at a post office in South San Francisco. They had been shipped from China, and contained no active oseltamivir.

Roche’s consumer guide warns among other things, to steer clear of Tamiflu bought over the Internet and to obtain the drug only through a pharmacy and with a valid prescription.

It also gives a clear description of the product - Tamiflu comes in a white cardboard box, which contains a single blister package with 10 capsules, each of which is a distinctive yellow and light grey colour and printed with the words ‘Roche 75mg’.

"Consumers worldwide can be assured that Roche is doing everything possible to protect the integrity of Tamiflu, and indeed all its products," said George Abercrombie, president and chief executive of Hoffmann-La Roche, Roche's US subsidiary.

Counterfeit drugs currently account for around 10% of the global pharmaceuticals market, according to the World Health Organisation. The fakers tend to concentrate on high-profile products with strong consumer awareness and demand, with other popular targets being Pfizer’s top-selling cholesterol lowerer Lipitor (atorvastatin) and erectile dysfunction product Viagra (sildenafil), although no product seems to be immune.