Last month fresh reports emerged of cases of blindness, usually temporary, in users of Viagra. The male impotence drug is widely traded on the internet and used without a doctor's prescription.

The dangers of online trading are set to be highlighted later this year when a case alleging counterfeit manufacture of two types of "lifestyle" drugs comes to court.

The case follows a raid on a warehouse in Wembley, north London, in which thousands of allegedly counterfeit pills were seized. The Medicines and Healthcare Regulation Agency, which is bringing the prosecutions, said people need to be careful when buying drugs from informal sources.

Sara Coakley, of the agency, said: "There's nothing wrong in buying medicines through the internet, you can deal with legal online pharmacies. But people need to be careful."

She said that expatriates - known to be particularly heavy e-buyers - should be extra careful.

One drug allegedly involved in the Wembley case is the erectile-dysfunction product Cialis, known as the "weekend romance pill" because it is effective for longer than Viagra. The other drug said to be involved is slimming agent Reductil.

Ms Coakley said: "About 20pc of people who go to their doctor for Viagra or Zyban are refused for medical reasons." If those individuals sought supplies from any source - legitimate or otherwise - they were at risk, she added. For instance Zyban, an anti-smoking pill, should not be taken with alcohol. Erectile dysfunction products are hazardous for those with heart trouble.

The bulk of cheap drugs are either generic copies of the brand product or the original product which has been bought in a country where the drug is sold cheaply.

Price differences occur everywhere. Prices in Canada are half those in the USA. Aids drugs sold at production cost in Africa following an outcry over millions there dying untreated, have sometimes not reached their target. They have been sold back to traders by corrupt officials. They then resurface on the net.

The market rewards the patient who shops around. EU member states decide prices on very different criteria and a variety of pricing mechanisms inevitably leads to wide price differences. Expatriate e-buyers can clearly cash in. But at what risk? "The greater the number of hands a product passes through, the easier it is for criminals to insert a counterfeit," said Andrew Curl, the director general of the Association of the British Pharmaceutical Industry.

Of course the association, representing the big-name manufacturers of branded drugs, has an interest in talking up the dangers of internet pharmacy.

But as long as massive price differentials exist the official price of products is going to be undercut.

That opens an opportunity for the careful internet buyer.