Half of men buying the impotence drug Viagra online are getting counterfeit tablets, study findings suggest.
As many as 50% sold could be fake
Dr Nic Wilson from the University of London tested Internet-sold samples using a new technique that accurately spots the ingredients of tablets.
She told the British Pharmaceutical Conference in Manchester how many authentic-looking tablets were fakes.
Drug giant Pfizer, which manufactures Viagra, is conducting its own investigations into the fake copies.
Dr Wilson used a technique called near infrared (NIR) microscopy which provided a detailed picture of what was in each tablet.
This technology is similar to the older method called NIR spectroscopy but gives much more information.
Dr Wilson explained: "A counterfeit tablet may contain lactose as an ingredient in the bulk tablet, whereas the authentic tablet does not.
"NIR spectroscopy could only show that the tablet is different, while NIR microscopy could actually identify the likely presence of lactose."
Many of the samples tested contained less of the active ingredient sildenafil than authentic Viagra.
They also contained different components from the bona fide Viagra.
Dr Wilson said: "We don't know that 'wrong' components will be harmful, but the user runs the risk of poor quality and possible toxicity, not to mention the fact that there is a high probability that the tablets have no clinical effect."
She said NIR microscopy should help regulatory authorities monitor the movement of counterfeit tablets.
It is the job of the Medicines Healthcare products Regulatory Authority to investigate any reports it receives of websites under its jurisdiction which appear to be in breach of regulations regarding advertising, or sale and supply of medicines.
In 2003, counterfeit Viagra with an estimated value in excess of £2.35m was seized.
Pfizer said it welcomed the research and was also analysing the samples.
A spokesman said: "We do not recommend that anyone obtains any prescription-only medicine online without seeing a doctor.
"Without knowing what ingredient is in a counterfeit medicine a patient could be putting their health at risk.
"It is important for men to see their doctor if they have erectile dysfunction as it may be an indicator of a more serious condition.
"Viagra may not be suitable for all men and there are also some men for whom sexual activity is not considered safe or for whom an oral treatment may not be appropriate."