Their study was due to appear in the December edition of the Annals of Internal Medicine, but the journal's editors posted it on the Internet as a matter of public interest.

The researchers focussed on two drugs commonly offered over the Internet - Viagra, for impotence, and Propecia, a treatment for male baldness.

They then compared the prices - not including delivery charges - with those on offer in chemists in the Philadelphia area.

To compare the cost of consulting a doctor, they used data from Medicare, the US health agency.

Unaccountable sites

The researchers, based at the University of Pennsylvania, were also alarmed that they found nine sites based outside the US offering the drugs without asking for a prescription.

Online health care carries risks

They looked at 46 sites - only five of which clearly stated where their businesses were based.

Professor Bernard Bloom, who led the study, said: "The Internet holds great promise for improving access to and quality of health care.

"But you often don't know what's in the drug you order, who made it, where it came from, where the online physician is who is prescribing the drug or even if he is a physician at all."

Consumer risks

The study is being published alongside an editorial written by four health specialist from the US Food and Drug Administration.

They said that while buying drugs over the Internet was convenient for elderly, disabled or physically isolated people, the American Medical Association regarded it as bad practice for sites to offer prescription medicine on the strength of an online questionnaire.