If you're about to buy some of those little blue pills, hold off: the price of Viagra is about to soften dramatically. That's because the UK patent for the erectile dysfunction drug, made by Pfizer, is up from today.

There are a host of copycat erectile dysfunction versions selling from 80p a pop - the price Viagra could now flop to.

What's up?

The loss of Pfizer's patent protection means rival operators can churn out generic versions of the blockbuster drug under its original chemical name, sildenafil.

A new price tumble - Viagra has in the past sold for £10 a pill - looks inevitable, though Pfizer has still got the US market, with patent protection extended there to 2020.

Pfizer will now launch a cut-price UK version - Sildenafil Pfizer - to cling to its market share this side of the Atlantic. The Viagra patent cut-off should save the NHS a load of prescription cash too (between 2011 and 2012 it's thought there were 1m plus UK prescriptions).

Hard time for investors?

A similar process was underwent by Pfizer's cholesterol-beating drug Lipitor last year, after coming off patent protection in late 2011; sales plummeted from almost £10bn to well under £4bn.

As a brand Viagra still has pulling power. Viagra sales last year - $2.05 billion - were worth a fraction of Pfizer's total revenues for the year. So Pfizer investors should have some excitement left.

Growth chart

Bear in mind that the R&D costs for a drug like Viagra are, well, huge. The US pharmaceutical industry lobby group, Pharmaceutical Research and Manufacturers of America (PhRMA) claims developing a new drug and hauling it to market can take up to 15 years and $500 million in costs.

And plenty of R&D and drugs fall by the wayside. If companies don't have the incentive of future profits, why should they risk investor cash developing new drugs? Meanwhile if Pfizer is successful still selling Viagra online direct, it could be a model copied by other manufacturers.

A growth model, even.