Dr Nic Wilson from the University of London tested generic viagra using a new technique that accurately spots the ingredients of tablets

She told the British Pharmaceutical Conference in Manchester how many authentic-looking generic viagra tablets were fakes.

Drug giant Pfizer is conducting its own investigations into the fake copies.

Half of men Purchase Viagra Online are getting counterfeit tablets, study findings suggest.

Dr Wilson used a technique called near infrared (NIR) microscopy which provided a detailed picture of what was in each tablet generic viagra. This technology is similar to the older method called NIR spectroscopy but gives much more information.

Dr Wilson said: We don't know that 'wrong' components of generic viagra 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 generic viagra tablets.

It is the job of the Medicines Healthcare Products Regulatory Authority (MHRA) to investigate any reports it receives about Viagra under its jurisdiction which appear to be in breach of regulations regarding advertising, or sale and supply of medicines.

Counterfeit generic viagra with an estimated value in excess of £2.35m was seized.

Pfizer, manufacturers of Viagra said it welcomed the research and was also analysing the samples of buying generic viagra.

A spokesman said: We do not recommend that anyone obtains any prescription-only Viagra without seeing a doctor.

Without knowing what ingredient is in a counterfeit medicine viagra 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.

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