The anti-impotence drug Viagra will be available on the High Street without a prescription from 14 February.
Users will pay £50 for four pills
Boots the chemist is introducing a trial scheme at three of its branches in Manchester.
Men aged between 30 and 65 will be able to buy four pills for £50 after a consultation with the pharmacist.
But Dr Jeff Hackett, chairman of the British Society for Sexual Medicine, said many men were entitled to the drug on the NHS, without paying.
"We have some regulations at the moment that allow a large number of patients to get the drug free on the National Health Service," he said.
"One of the problems for pharmacists will be to identify these patients who actually shouldn't be paying who are legally entitled to get it free and that's quite a challenge."
Men seeking the drug from the pharmacist will have to undergo some basic medical tests, and anyone wanting a repeat prescription would have to consult a doctor.
Boots pharmacist James Longdon said the men would have an hour-long consultation, including blood pressure, blood sugar and cholesterol checks.
He added that they would also be made aware that men with certain medical conditions could obtain the drug free of charge with a prescription from their GP.
Boots claims that only 10% of the three million men who suffer from impotence are being treated.
It said offering Viagra without a prescription could help to improve those figures.
The chemist also claimed the move would be a good way to monitor men's health, as erectile dysfunction was often a marker for a more serious underlying medical condition.