The company has just alerted pharmacists in the US to a fresh wave of counterfeit versions of the product, adding impetus to its calls for more stringent controls on the supply chain for pharmaceuticals.
Last year, a US drug distributor was forced to recall 1.8 million tablets labelled as Pfizer's Lipitor (atorvastatin) - a cholesterol-lowerer that is the top-selling drug in the world with annual turnover of $10 billion (€8.2bn) - because of counterfeiting.
In response, Pfizer has adopted all sorts of measures to try to thwart the fakers, including unveiling a new label for Viagra that is written in a special ink that changes its colour from purple to blue depending on the angle from which it is viewed. However, the latest case applies to older stock, which does not use the new ink, according to the company. This old inventory will gradually disappear from circulation between now and the end of the year.
The company is taking stringent measures with drug wholesalers, insisting that they only buy Pfizer drugs from Pfizer or other approved sources or risk being denied rights to distribute its products, and has asked other companies and regulators to increase their efforts in tackling the issue.
Meanwhile, in Europe, Pfizer is adding tamper-evident packaging across its range in a bid to make it harder for counterfeiters to copy its drugs. And in common with other drugmakers it is also exploring the use of various track-and-trace technologies, including radiofrequency identification (RFID), to shore up security in the supply chain.
But in the meantime counterfeit products are still reaching pharmacies and patients, placing them at risk of not receiving proper treatment or suffering serious side effects, says the company.
Pfizer has taken the step of publishing pictures of the latest crop of fake Viagra, identified in two pharmacies in California, to provide a clear example of just how sophisticated the counterfeiters have become.
Both Pfizer and the US Food and Drug Administration (FDA) are analysing the fake pills to see if they pose any risk to human health, and note that the definitive way to tell the difference is to split the pills in half. The fakes will have blue speckling in the cross section, while genuine Viagra will have a plain white core.
According to US health officials, one in 10 medicines sold worldwide is fake, providing about $32 billion in annual sales to the counterfeiters. In developing countries the proportion can be as high as 25 per cent.
In 2003 there were 22 newly-opened FDA cases associated with counterfeit drugs, compared with six in 1997.
The potential profits for counterfeiters are perhaps best exemplified by the following case, which occurred in the US. A group of Florida counterfeiters purchased 11,000 boxes of Amgen's Epogen (epoetin alfa), which is used to boost red blood cells and treat anemia in people with chronic renal failure, for $2.5 million. They altered and repackaged the drug before selling it to wholesalers for nearly $49 million, for a $46.5 million profit.