Doctors issued a new warning on the dangers of buying drugs over the internet after it emerged one in four has treated patients made ill by them.
People face a "minefield" when it comes to purchasing online, with many sites offering fake medicines and promoting online doctors and advisers who have no professional qualifications.
According to the Royal Pharmaceutical Society of Great Britain (RPSGB), more than two million people in the UK regularly buy medicines over the internet.
It says that while some online pharmacies are legitimate, in many cases the medicines purchased from websites are counterfeit, substandard or unapproved new drugs - which all put patients at risk.
People commonly buy drugs on the internet for a range of conditions, including erectile dysfunction and anti-depressants.
The poll of 423 doctors for GP newspaper found 25% have treated patients for adverse reactions caused by medicines bought over the internet. A further 8% think they may have treated somebody but were unsure. Overall, 85% said online pharmacies needed to be more tightly regulated.
Dr Bill Beeby, prescribing lead for the British Medical Association's GPs' committee, said: "When it comes to buying drugs on the internet, it is a minefield.
"People just don't know what they'd be getting. I wouldn't advise any of my patients to go down that route."
The RPSGB has created an internet pharmacy logo to help people work out if a website is being operated by a bona fide pharmacy in the UK. All UK pharmacies, including those offering internet services, must be registered with the RPSGB.
A statement from the Medicines and Healthcare products Regulatory Agency (MHRA) said: "People are advised that they should only take prescription-only medicines after an appropriate consultation with their GP. Only healthcare professionals can take into account risks and benefits associated with every medicine."
