The little blue pill that has restored the sex lives of millions of men could soon be transformed - into a faster-acting skin patch.

It could cut the time Viagra needs to start working from up to an hour to a matter of minutes, and increase its effectiveness for longer than its current ten hours. 

The patch, which could be available in a few years, may also reduce side-effects of the pill such as indigestion, headaches, migraines, disturbed vision and muscle pain.

It is, however, expected to cost £10 ($28) while the pill is a mere 30p.

In a scientific breakthrough, researchers have developed a way to shrink the drug used to make Viagra, sildenafil citrate, into nanoparticles small enough to pass through the skin and enter the bloodstream. 

Viagra has helped to transform the treatment of impotence - or erectile dysfunction. But the 25mg pill needs to be taken at least 30 minutes to an hour before sex, and has no effect for around one in three men. 

Similar drugs such as Cialis trigger erections more rapidly but still carry the risk of side-effects. 

The only other option is to inject drugs straight into the penis, or use a pump to manually increase its blood supply. Neither is popular. Attempts to get the drug directly into the bloodstream have been hindered by the fact that most medicines are made of molecules too big get through the skin. 

But scientists from Cairo University and King Abdulaziz University in Jeddah, Saudi Arabia, made a patch with a thin film containing minute particles of the drug coated in a thin layer of fat to help with absorption, and a chemical that improves penetration of the skin. 

After a prototype was tested on rats, scientists said the Viagra successfully passed through the outer and inner layers of the skin. 

In a report in the journal Drug Design, Development and Therapy, they said: "This could be a promising delivery system." 

Dr Geoff Hackett, of the British Society of Sexual Medicine, said it could be popular with some men, but could prove to be expensive when tablets cost 30p each. 

He added: "I cannot really see how a patch which is likely to cost around £10 is going to replace a pill that is a fraction of the price."