People who buy medicines over the internet could be unwittingly putting their health at risk, warn UK doctors.
Drugs bought online can be fake
Some drugs are fake and contain ingredients bearing little resemblance to the medicine named on the bottle, the Sunderland team told the Lancet.
Even if patients get the right drug, there is a risk of unchecked side effects and dangerous interactions.
The team reports the case of a woman who damaged her vision with oral steroids bought online from Thailand.
The 64-year-old woman had taken the drug for four years after making a self-diagnosis of chronic fatigue syndrome.
In February 2006, she went to the accident and emergency department of the Sunderland Royal Infirmary because she had noticed her eyesight failing for the previous six months.
The doctors who examined her discovered she had glaucoma and cataracts, caused by the steroids she had been taking.
Prompted by this case, the doctors, Mr Scott Fraser and Dr Philip Severn, did an internet search and found they were able to find 1,000 tablets of the same steroid - prednisolone 5mg - online for £25.23.
Mr Fraser said that even though the treatment she had been taking would have been correct, the dose was too high and she had not been monitored for side effects.
Had she been monitored, the problem would have been spotted earlier and her complications might have been avoided.
Writing in the Lancet, the researchers said: "The expansion of the internet is relentless and, from the perspective of patients seeking information, in the main positive.
"However, the online availability of controlled and uncontrolled drug therapies needs to be carefully monitored."
Mr Fraser added: "If you look online it is extremely easy to get hold of anything you like. Obviously, you do not need a prescription.
"If you are going to access medicines on the internet you are gambling with your own health."
He said doctors needed to be more aware of the issue and to make sure that they asked patients whether they were taking any medicines purchased over the counter or online.
A spokesman for the Medicines and Healthcare Products Regulatory Agency said: "Tackling the illegal sale and supply of medicines over the internet is a priority for us.
"The MHRA is currently investigating 117 internet-related cases where medicines legislation has been potentially breached.
"Some of these investigations have already resulted in successful prosecutions.
"Patients are advised that they should only take prescription-only medicines after an appropriate consultation with their healthcare professional whose role it is to take into account associated risks and benefits."