A 'seal of approval' scheme is needed to advise the public which websites on complementary treatments for cancer they can trust, say researchers.
Information varied in quality
An analysis of 32 websites by a team from Plymouth Peninsula Medical School found the information provided was of extremely variable quality.
Many endorsed unproven therapies, and some were even dangerous.
The researchers say cancer charities and research groups should vet the sites, and develop a kite-mark system.
Overall, the survey showed that most of the websites did provide valuable and reliable information, especially for the prevention of cancer.
Little effect shown:
Some evidence of benefit:
However, some issued information on complementary and alternative medicine (CAM) that was misleading.
Researcher Professor Edzard Ernst, an expert on complementary medicine, said: "These websites promoted and discussed CAM treatments for which no compelling safety and efficacy data exist.
"Generally speaking, the 'cancer cures' discussed on these websites are not supported by good scientific evidence.
"Other sites are outright dangerous as they advise patients against using conventional therapies."
Some good sites
Professor Ernst said health websites could provide important help to people - but it was important to raise awareness of the variable quality of the available information.
The researchers had particular praise for two sites, Quackwatch and Bandolier, which they said were both of excellent quality.
However, three sites were rated as dangerous because they discouraged patients from using conventional cancer care.
Of the total CAM therapies advertised, 118 were for cure, 88 for palliative/supportive care and 59 for prevention.
The researchers carried out extensive literature searches in scientific and medical databases to assess the benefits of some of the most commonly featured therapies. They found little evidence to support the claims made on their behalf.
Professor Ernst said: "Our analysis has shown that many websites are not as reliable as one would hope.
"In the interests of cancer patients we should find ways of minimising the potential harm such misinformation may cause."
Dr Richard Sullivan, of Cancer Research UK, said: "More and more cancer patients are using CAM in parallel with their conventional treatments.
"There are some good websites to guide patients through CAM, but too many are either poor quality or frankly misleading.
"There is a real need to provide good quality information to patients and doctors about the potential and pitfalls of CAM in all its myriad forms."
Peter Cardy, chief executive of the charity Macmillan Cancer Relief, said: "Complementary therapies have become a significant feature of healthcare practice in the UK and there is a need for good quality information on the internet.
"As part of Macmillan's service to patients, carers and professionals we already examine bonafide websites for cancer patients and check the quality of their information.
"Evidence suggests that many people find that complementary therapies help them cope with the experience of cancer when used alongside conventional treatments.
"Patients report reduced anxiety, less depression, and an improved sense of well being.
"Macmillan recommends that people with cancer should discuss the use of complementary therapy with their doctor or nurse before embarking on any treatment and should reject any advice from a website that recommends abandoning conventional treatment in favour of alternative cures."
The study is published in the journal Annals of Oncology.