Counterfeiters are playing on public fears of an influenza pandemic and producing bootleg versions of Roche’s flu drug Tamiflu (oseltamivir), according to US custom officials.

The US Customs and Border Protection Service seized shipments of counterfeit Tamiflu at an airmail facility in San Francisco, and anticipate that many more will be on the way. The fake drugs contained only faint traces of the active ingredient, and was believed to have been ordered over the Internet and shipped from Asia, according to the US Food and Drug Administration.

Tamiflu is considered to be the drug of choice for treating patients infected with avian flu strains such as H5N1, and governments around the world are stockpiling the drug amid fears that a long-overdue flu pandemic is just around the corner.

Roche has been racing to raise its production capacity to meet demand, but while it has said it will have more than enough to go around by 2007, surging demand has made supplies more limited in the near term. In October, the firm halted shipments of Tamiflu to the USA after an unusually high number of prescriptions sparked fears that consumers were hoarding the product. Similar actions were taken in Canada, Germany and Switzerland.

High public demand swiftly elevated Tamiflu to the ranks of those other much-copied products – such as Pfizer’s erectile dysfunction drug Viagra (sildenafil) and cholesterol-lowerer Lipitor (atorvastatin).

Earlier this year, the open-access health journal PloS Medicine published a study suggesting that up to 15% of all drugs sold worldwide – worth around $35 billion – are fakes. This estimate – based on communications with regulatory authorities, is more than twice the 6 per cent figure put forward by the World Health Organisation.

And in September a US think tank, The Centre for Medicines in the Public Interest, released a report projecting counterfeit drug sales to reach $75 billion in 2010, which it said was a 92% increase over 2005 levels.