Doctors have warned of the perils of buying medicines over the internet.
One in four GPs polled said they had treated patients for adverse reactions to medicines bought online.
A further 8% suspected they had treated side-effects of internet-bought drugs, the snapshot survey of 420 doctors carried out by GP magazine found.
Pharmacist leaders urged the public to be aware of the risks of internet medicines and use bone fide sites which require a prescription.
Overall, 85% of the GP respondents want online pharmacies to be more tightly regulated.
The Royal Pharmaceutical Society of Great Britain has estimated that two million Britons regularly buy drugs over the internet, a figure which is thought to be on the rise.
Anyone who self medicates and buys their medicines from internet sites could be in danger of receiving counterfeit or substandard medicines
This includes those who purchase medicines prescribed by their doctor from a legitimate online pharmacy.
But it is common for illegal internet pharmacies to promote “lifestyle” medicines, such as those for erectile dysfunction, slimming pills, hair-loss treatments, and anti-depressants.
Medicines regulators said these drugs are in high demand due to the perceived “embarrassment” factor and the fact that a GP may not prescribe them to the particular patient for one reason or another.
The Medicines and Healthcare Products Regulatory Agency said increasingly people are purchasing drugs that may not be available via the NHS due to cost-restrictions.
Dr Sarah Jarvis, a GP in west London and spokeswoman for the Royal College of GPs, said: “Surveys looking at many online medications suggest that the proportion of counterfeits is enormously high and that many of them contain very worrying ingredients.”
Dr Bill Beeby, prescribing lead for the British Medical Association’s GPs committee, said he would not advise any of his patients to buy medicines online.
“When it comes to buying drugs on the internet, it is a minefield. People just don’t know what they’d be getting.”
The Royal Pharmaceutical Society runs a logo scheme for people to identify legitimate online pharmacies and they are also launching an awareness campaign through pharmacies to warn people of the dangers of counterfeit traders.
BONA FIDE PHARMACY SITES
Should carry the Internet Pharmacy Logo
Should provide address and pharmacist registration details
Should not offer prescription-only medicines without prescription
Should ask questions
David Pruce, director of policy at the society advised only purchasing drugs from an internet site connected to a “bricks and mortar” pharmacy.
He added: “It is never a good idea to take a prescription medicine without a valid prescription.
“The medicine may not be suitable for you and could result in unpleasant side-effects or serious health risks.”
The MHRA, which supports the logo scheme, can only regulate sites operating in the UK but said tackling the illegal sale of medicines online was a “priority”.
“Only healthcare professionals can take into account risks and benefits associated with every medicine,” a spokesman said.
“Anyone who self medicates and buys their medicines from internet sites could be in danger of receiving counterfeit or substandard medicines.
“At best these will be a waste of money, at worst they can kill.”