Internet Web sites are recommending unproven complementary medicines for cancer that could interfere with conventional treatments and be dangerous or deadly, a leading expert said on Monday.

Professor Edzard Ernst analysed 32 Web sites and found many recommended treatments not supported by scientific evidence.

“If it sounds too good to be true it probably is,” said the chair of complementary medicine at the Peninsula Medical School at the Universities of Exeter & Plymouth in southern England.

Not everything natural is risk free, he told reporters.

The Web sites recommended 118 cancer cures, 59 treatments to prevent the illness and 88 for palliative care. Some sites provided information that discouraged patients from using conventional treatments for cancer.

“A significant proportion of the Web sites are a risk to cancer patients,” he said. “There is no good evidence that any complementary treatment can prevent cancer.”

Ernst said that for example shark cartilage is recommended for cancer on some Web sites but “there is not a shred of evidence that it helps cancer patients.”

But some sites provided very good information about complementary treatments for cancer and other illnesses, which he believes should be marked so patients know they are getting the best information.

“One way forward would be to flag up these Web sites for patients because how is a patient going to know this is reliable and this is not reliable?”

A seal of approval for a Web site by national cancer societies would remove any doubts the patient may have, Ernst added.

The benefits of some therapies, such as acupuncture for back pain and nausea, St. John’s wort for mild depression, and garlic to reduce cholesterol levels, have been shown to work but only a small amount of complementary therapies have been rigorously tested, according to Ernst.

In a survey he conducted of 2,600 patients who had been prescribed a blood-thinning drug, he found that 9 percent were also taking a herbal medicine that could interfere with the effectiveness of the conventional drug.

But the majority of them had not told their doctors about the herbal medication.

Ernst believes complementary therapies have a place alongside conventional treatments but that more research and scientific evidence is needed.