Cancer patients who choose alternative medicine over standard, proven cancer treatments are more likely to die, researchers reported Thursday.

Complementary medicine did no apparent harm if people used it alongside conventional surgery, chemotherapy and radiation, the researchers found. But when people opted out of proven treatments to choose herbs, homeopathy or other alternative treatments, they were twice as likely to die of their cancer.

Doctors and the Food and Drug Administration have warned for years that unproven treatments may lure patients away from legitimate therapy that can save their lives. But it’s one thing to say that a treatment has not been shown to help, and it’s another to show for sure that it doesn’t.

“It’s shocking, the lack of comparative data that’s out there,” said Dr. James Yu of the Yale Cancer Center.

Yu and his team set out to show whether people who use complementary therapies are hurting their chances of surviving cancer.

They looked at the medical records of nearly 2 million cancer patients. Not many admitted using complementary medicine, or had it noted in their records, but 258 did. Their cases were compared with the cases of more than 1,000 patients who did not use complementary or alternative medicine.

Those who chose alternatives such as herbs, homeopathy, naturopathy or Chinese medicine were also more likely to refuse at least some standard cancer treatment, Yu’s team reported in the Journal of the American Medical Association’s JAMA Oncology.

Seven percent of those who chose complementary treatments refused surgery, for instance, they found — compared with just 0.1 percent of patients who just went with standard treatment. More than a third declined chemotherapy or hormone therapy, and half refused radiation therapy.

“If you could cure cancer with baking soda, who wouldn’t want to do that?"

Those who chose alternatives tended to be younger women with more education and more money, they researchers found.

The message to doctors is clear, Yu said.

“When a patient is using complementary medicine, make sure you are really listening to that patient and their needs because they are more likely to refuse treatment,” he told NBC News.

Cancer patients have unrealistic views of the value of complementary therapy, Yu’s team wrote.

“Approximately two-thirds of patients with cancer believe that complementary medicine will prolong life and one-third expect it to cure their disease,” they wrote.

Yu is sympathetic.

“I think it is a very human response — when faced with a treatment that is potentially disfiguring or toxic with real long-term effects — to wonder whether there is another way,” Yu said.

“I totally get it. Unfortunately, people who have cancer are very vulnerable to unscrupulous or perhaps well-meaning but ill-informed practitioners who offer a nonmedical treatment that is unproven but on the surface seems very desirable.”

It is not unreasonable to hope, Yu said, and oncologists should listen to patients’ concerns.

“If you could cure cancer with baking soda, who wouldn’t want to do that? Or if you could cure cancer with healing power crystals or positive thinking, who wouldn’t want that? I completely understand and empathize with patients,” he said.

And if a complementary treatment helps a patient feel better, alongside proven therapy, Yu said he is all for it.

“There is a broad spectrum of complementary medicine used by patients with cancer, including herbs and botanicals, vitamins and minerals, traditional Chinese medicine, homeopathy and naturopathy, as well as specialized diets,” his team wrote.

“Past research has shown that complementary therapies such as massage, acupuncture, yoga and meditation can improve quality of life. Thus, it is estimated that between 48 percent and 88 percent of patients with cancer have reported the use of complementary and alternative medicine as part of their therapy.”

The research team did not look at specific, individual alternative therapies but said they can include probiotics, Ayurvedic medicine, deep breathing, yoga, tai chi, qi gong, acupuncture, chiropractic or osteopathic manipulation, meditation, massage, prayer, special diets, progressive relaxation, and guided imagery.

“We need to do a better job of listening to our patients,” Yu said. “The use of complementary medicine, I think, is very common and oncologists need to know that.”

The National Institutes of Health’s National Center for Complementary and Integrative Health has an app for people who want to check out the research on alternative therapies.

The center says people who use alternative therapies may be afraid to tell their doctors, and notes that some herbal products, such as St. John’s wort, can interfere with cancer treatments.