This story was updated Dec. 18, 2008.
When 45-year-old Melissa Offenhartz was diagnosed with breast cancer last May, she went straight to her computer, CBS News medical correspondent Dr. Jon LaPook reports.
"I Googled everything from initial things I found on my mammogram report to little phrases, maybe, that my doctor had with me in conversation," she said.
As she surfed, she was soon lost in a sea of phony remedies. One site says: "Cancer can be cured and prevented naturally and scientifically."
And some seductive promises, like: "dynamic therapies" and "patient victories are happening every day."
Some Web sites sold anti-oxidants to boost her immune system. It sounded reasonable, but when she checked with her doctor, she learned she was about to make a big mistake.
"When in fact you're on chemotherapy, anti-oxidants are not something you should be taking, because anti-oxidants protect cells, and we don't want to be protecting those cancer cells from chemotherapy," she said.
Offenhartz was nearly tripped-up by bad medical advice on the Web … and she's a nurse!
"There are a lot of people out there trying to take advantage of cancer patients," said Barrie Cassileth, the chief of integrative medicine at Memorial Sloan Kettering cancer center.
Cassileth has seen cancer patients harmed - even die - after being duped by fake online cures.
"I've seen patients go to clinics where they receive oxygen therapy for twenty thousand dollars and they have given up their children's college tuition fund, they've sold their homes, and they do this and it has zero value," she said.
Even treatments debunked decades ago, like Laetrile, made from fruit pits, are still easily bought online.
LaPook went to one of the many Web sites offering it, and said: "I'm ordering Laetrile for $19 plus $8.50 shipping."
About a week later, his Laetrile arrived.
"A few easy clicks of the mouse and I've got my cancer cure. Only problem is ... it's bogus," LaPook said.
"Laetrile was banned by the FDA," Cassileth said. "It's a useless treatment."
Regulators are cracking down on companies making fraudulent claims, and the FDA has sent out 35 warning letters citing more than 260 products, including Laetrile.
But officials say it's almost impossible to keep up with a marketplace so vast.
"We are constantly scouring the internet for illegal companies. Unfortunately, as soon as we can shut them down more pop up," said the FDA's Dr. Jason Woo.
So, here are some basic rules for searching medical information.
Some doctors say don't surf the Web at all. But the temptation can be irresistible.
"When I went to see my oncologist, she and her nurse actually both recommended that I stay off the computer," Offenhartz said.
Second Opinion: Medicine Online
Read part II | Part III
LaPook asked her: "So, what did you do?"
"I went online and searched!" she laughed.
The best advice is to include your doctor. Together you can figure out what information to trust and how it applies to you.
EDITOR'S NOTE: An earlier version of this story online cited a testimonial from a website featuring a purported quote from Ralph W. Moss, Ph.D., which read "Conventional cancer therapy is so toxic and dehumanizing that I fear it far more than I fear death from cancer." The same quote appears on many websites patients may encounter while searching online medical advice. Moss contacted CBS News to say he never made the statement. CBS News confirms it was in fact made by Dr. Julian Whitaker in 1995 and not by Moss. We regret the inaccuracy.