One third of people with cancer are using complementary and alternative medicines (CAMs), reports the University of Texas Southwestern Medical Center in a new study published in the journal JAMA Oncology. But nearly a third of those don’t let their doctors know.

According to the National Cancer Institute, CAMs are medical products and practices that are not a part of standard care. Complementary treatments are those used along with such care, while alternative treatments are used instead of traditional therapies. These medicines are usually taken to help ease the side effects of cancer treatment or treat cancer.

Among the more than 3,000 participants who reported a history of cancer, 1,023 (33.3%) reported using complementary and alternative medicine. The most commonly used CAMs were herbal supplements (35.8% of participants), followed by chiropractic or osteopathic manipulation (25.4%), massage (14.1%) and yoga/tai chi/qigong (7.6%). Young people and women were more likely to use CAMs.

Nearly a third (29%) of those who used CAMs, however, didn’t share their use of such treatments with their doctors. Reasons included not being asked or feeling that their doctors didn’t need to know.

But this is of concern to doctors like Nina Sanford, MD, an assistant professor of radiation oncology at the University of Texas Southwestern Medical Center in Dallas, especially with regard to herbal supplements, the exact contents of which, Stanford notes are often unknown.

“Unless we know what’s in them, I would recommend patients avoid using them during radiation because there’s likely not data on certain supplements, which could interfere with treatment,” said Sanford. “With radiation specifically, there is a concern that very high levels of antioxidants could make radiation less effective.”

Patients are encouraged to share supplemental use with their providers. They are discouraged, however, from using alternative medicine instead of traditional treatments, as this practice can be quite deadly.

Sanford points to Apple founder Steve Jobs. Jobs, who died in 2011 of pancreatic cancer, had relied only on special diets, acupuncture and other alternatives when he was first diagnosed. He didn’t seek traditional treatment until later in his cancer battle, when it might have been too late.

The study authors advise anyone being treated for cancer to discuss any complementary medicine they are taking or considering, especially supplements, with their doctors. Many major medical centers have complementary medicine divisions (sometimes referred to as integrative care) to help people being treated for cancer use practices, including yoga and meditation, that relieve stress and help manage the side effects of cancer treatment.

For related coverage, read “Complementary Therapies,” “40 Percent of Americans Believe Alternative Therapies Can Cure Cancer” and “Cancer Patients Using Alternative Therapies Are More Likely to Die.”