In some cases, they may be sentencing themselves to death by rejecting conventional treatment in favour of alternative therapies, it was claimed.
A new study identified 32 cancer treatment websites that attracted tens of thousands of "hits" every day from around the world.
But not one of the numerous treatments and approaches they advocated could be shown to cure or prevent cancer.
In 3% of cases, the sites actively discouraged patients from using conventional treatments, and 16% did the same indirectly through the information they provided.
Three sites heall.com and healthy.net, based in the United States, and worldwidehealthcenter.net, based in the UK and Cyprus were judged to offer advice that was potentially harmful to patients.
Professor Edward Ernst, from the University of Exeter, who co-led the study, said: "This was to us quite an eye-opener and pretty scary stuff.
"Our conclusion was that a significant proportion of these websites are actually a risk to cancer patients.
"We found that between these 30-odd sites, 118 different cancer "cures" were recommended, each a complementary treatment which claimed to be able to cure cancer. None of these 118 can be demonstrated to cure cancer."
He said a further 59 preventative treatments were recommended, but again there was no evidence that any of them worked.
Two prime examples of bogus cancer treatments were shark cartilage and "laertrial", which is made from apricot stones.
In the first case, the demand for ground-up shark fins had brought two species of shark close to extinction, said Prof Ernst. There was "not a shred of evidence" that it helped cure patients.
Laertrial was heavily promoted in America in the 1970s. After being driven underground by studies showing that it could be harmful, a conspiracy theory arose claiming that drug companies had wanted to kill off competition from the treatment.
Since then, it has resurfaced and could now be found on hundreds of websites.
There had been reported cases where cancer patients had died as a result of using products promoted on the internet, said Prof Ernst.
He said: "Not everything that is natural is risk-free. People should use their common sense and think twice about the motives of these websites.
"If it sounds too good to be true, it probably is. Don't believe ridiculous promises and claims. Use your brain; take some reasonable and responsible advice."
Prof Ernst is Britain's first and only professor of complementary medicine.
His research on websites appears in the journal Annals of Oncology.