People with sexually transmitted infections are putting themselves at risk by buying treatments over the internet, say researchers.
Many people prefer the privacy of the web
The University of East Anglia found less than a quarter of internet vendors gave information on potential side effects of their treatments.
A similar number also failed to say if their products would interfere with prescription medicines.
The study features in the journal Biomedcentral Public Health.
The East Anglia team also found that few vendors offered advice on whether their products might harm patients who were breastfeeding or pregnant.
Less than a quarter of vendors provided advice on how to avoid transmission of the STI or becoming re-infected.
Owing to the stigma associated with sexually transmitted infections, patients may prefer to hide their illness, and choose instead to try out internet remedies in the privacy of their own homes.
However, the researchers warned that it was important to get full advice on side effects, and on the need to treat sexual partners at the same time to avoid transmission or re-infection.
Some conditions, such as chlamydia and gonorrhoea, can lead to infertility if not properly treated.
The researchers found a total of 77 treatments from 52 different companies on the internet.
Rmedies were aimed at conditions such as genital warts, herpes, gonorrhoea, and chlamydia.
The average cost was around £50, with the most expensive being offered at £145.
Researcher Dr Roberto Vivancos said: "It may be easy and convenient to buy your own treatment on the internet but we think that people are taking significant risks in doing so.
"Treatment of sexually transmitted infections is not a simple one-off step of popping the pills or slapping on the creams."
The researchers also found that almost half of the products were claimed to be effective - but there was actually very little solid evidence from the vendors to back this up.
Dr Vivancos said: "Patients nowadays can easily buy kits from the internet to test themselves for sexually transmitted infections and they may be tempted to do the same when they need treatment.
"But the NHS offers a professional, confidential treatment service that is likely to be far better and cheaper than taking chances on an unproven internet remedy."
The Medicines and Healthcare products Regulatory Agency (MHRA) said it campaigned to raise awareness of the dangers of buying medicines from unlicensed internet sites.
In a statement, it said: "The public is 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."
The MHRA is currently investigating a number of internet-related cases where medicines legislation has potentially been breached.
A Department of Health spokesperson said increasing access to sexual health care services was a priority.
There were a wide range of options, including phoning NHS, and consulting government-backed online sources.
"We are not suggesting that patients should replace their consultation with a healthcare professional by simply looking online."