Image copyright SPL Image caption Viagra: the blue pill is now off-patent

There can be few drugs in recent decades which have become so embedded in popular consciousness as Viagra.

The diamond-shaped blue pill revolutionised treatment for erectile dysfunction.

Its launch 15 years ago caused a sensation - for a time it was the fastest-selling drug in US history.

One of my colleagues wrote that some doctors were complaining of cramp after writing out so many prescriptions.

And as for the jokes - from the witty to the puerile - Viagra was the drug that kept on giving.

But from midnight on Friday the drug came off-patent. That means Pfizer will face tough - I hesitate to write stiff - opposition from rival firms who will be able to sell a generic version under its chemical name sildenafil citrate.

It will mean the price falling from up to £10 a tablet to perhaps less than £1.

Keepy-uppy

Pfizer still has another seven years of patent protection in the US, but will be launching its own generic version of Viagra in the UK.

Sildenafil was created by scientists at Pfizer's Sandwich, Kent research facility. It started out as a treatment for high blood pressure but clinical trials revealed an unexpected side-effect of improved erections through increased blood flow to the penis.

The rest, as they say, is history.

The term "male impotence" was replaced by "erectile dysfunction" and a taboo area was normalised.

Football legend Pele was recruited to front a campaign about erection problems - although he told a newspaper that he could still do keepy-uppy and had no need for the pills himself.

"Viagra de-stigmatised erectile dysfunction and introduced the first treatment that was patient-friendly," said Jon Emms, Pfizer's UK managing director.

"It also encouraged men of a certain age - who didn't visit their GP - to talk their doctor; this led to other problems being identified and treated."

Viagra still produces around $2bn annual revenue a year for Pfizer, and is the company's sixth biggest product.

So what will happen now the sildenafil can be sold generically?

"The market size is likely to grow as the treatment becomes more affordable," said Dr Tom Brett, medical director of Lloydspharmacy Online Doctor.

"We also hope that the black market through illegal websites will shrink. Hundreds of websites have been shut down and 68,000 illegal doses were seized last year - although it's impossible to know the extent of the problem."

Most Viagra is bought privately. There are strict eligibility criteria for the treatment on the NHS.

Dr Brett said it is essential that men buy it from a trusted source so they can be sure the product is genuine and they can be assessed for any potential interactions with other medicines they are taking.