The claims seem remarkable. "If a person eats 6-12 apricot kernels per day, they will never have to worry about cancer," one site says. "Silymarin also seems to have anticancer properties. It can stop cancer cells from multiplying, kill cancer cells, and block their blood supply," says another.
They're also false, the Food and Drug Administration says. It's warned 14 companies to stop making claims about herbal products and other treatments marketed to treat or prevent cancer.
They cannot and some may be dangerous, the FDA says.
"These companies used slick ads, videos, and other sophisticated marketing techniques, including testimonials about miraculous outcomes," the FDA's Donald Ashley and Douglas Stearn wrote in a blog post.
"Often a single product was promoted as a treatment or cure for multiple diseases in humans and animals."
The FDA has listed the 14 companies on its website and detailed the false claims they have made about their products, which include herbs, tinctures, supplements, teas and salves.
The claims range from curing cancer to "detoxifying" the liver.
It's illegal to make such claims without proving they are true and going through the FDA's process for verifying them. Just putting a little disclaimer at the bottom of an ad saying the FDA has not verified the claims doesn't cut it, the agency said.
"Hoping to skirt the law on a technicality, some sellers made false claims and then in small print provided a disclaimer that their products are not intended to diagnose, treat, cure or prevent any disease," Ashley and Stearn wrote.
"Making such obvious claims and then saying later that you are not doing so might seem clever, but the technique does not comply with federal laws intended to protect public health."
The companies are taking advantage of frightened cancer patients and their loved ones, said consumer safety officer Nicole Kornspan.
And they make a lot of money doing it. Americans spend $30 billion a year on alternative therapies.
"Anyone who suffers from cancer, or knows someone who does, understands the fear and desperation that can set in," Kornspan said in a statement. "There can be a great temptation to jump at anything that appears to offer a chance for a cure."
They especially take advantage of people's tendency to believe that "natural" cures and treatments are better than those developed by commercial pharmaceutical companies.
But they're not only a waste of money — they can contain harmful ingredients.
For instance, "Everything Herbs" was advertising apricot kernels, which contain deadly cyanide. Apricot seeds were the basis for laetrile, an unproven but popular "alternative" cancer therapy sold online and in overseas clinics since the 1970s, despite much evidence it is worthless.
"Contained within these kernels is a very small amount of a substance called nitriloside amygdalin. It goes directly to a cancer cell, stings it, and kills it," the FDA quoted the "Everything Herbs" site as saying. The website was down for repairs when NBC News investigated and a call to the firm's phone number yielded a voicemail saying the entire company was on vacation.
Another site, DoctorVicks.com, sells a variety of herbs and supplements, including silymarin, also known as milk thistle.
"Milk thistle's main ingredient silymarin has been found to support the liver in wondrous ways. It can help fix the damage done to the liver by alcohol or Tylenol, and can protect the liver from future damage," it claims.
There is a compound that can help the damage done by Tylenol overdoses — it's called N-acetylcysteine (NAC) but it must be given immediately and by a medical professional. Some studies have suggested silymarin may be helpful but it has not been approved and the dose has not been established.
Dr. Vicks did not immediately respond to a request for comment from NBC News.
Related: FTC warns of bogus cancer cures
The FDA has given the companies 15 days to come up with a plan to comply with FDA orders. The agency can prosecute and fine the companies and even seek prison terms.
"The message to consumers is this: These products are untested. Some contain ingredients that may be a direct risk to your health," the FDA said.
"The ingredients may interact in a dangerous way with professionally-prescribed treatments. They are not a substitute for appropriate treatments. Using these products can waste your money, and, more importantly, endanger your health."