Pfizer's patent on Viagra has expired in the UK and other states in Europe, paving the way for the erectile dysfunction dysfunction market to be flooded with generics of the  blockbuster.

It has been reported that as 10-20 manufacturers plan to launch generic versions of Viagra (sildenafil) in the next few months, which will see the price tumble to around £1 a tablet from £6 or so at the moment. The world's largest generics player Teva will be among the firms marketing their version and Pfizer is also launching its own cheap alternative.

The little blue tablet has been hugely successful and last year brought in revenues of $2.05 billion worldwide, some 15 years after being launched. Half of that comes from the USA, where Pfizer still has patent protection for Viagra until 2020.

In the UK last year, 2.3 million prescriptions were written for the drug, costing the National Health Service £40 million. The lower prices will particularly benefit the estimated 150,000 men out of around 750,000 people  who take Viagra each year but pay for a private prescription.

A spokesman for the British Generic Manufacturers Association said that generic competition could see the price erode by as much as 90% providing significant cost savings to the NHS and wider availability to patients. "In the private market, patients can also expect to see a cost reduction although they may need to shop around to achieve the optimum price", he added, saying that in order to achieve savings patients "should seek to have their prescriptions written by the generic name [sildenafil].”