People who decide to tackle their cancer using only unconventional methods are likely to die sooner than patients who opt for conventional treatments, according to a new study.

People with various kinds of cancer who turned down treatments like chemotherapy or radiation in favor of alternative medicine were two to six times more likely to die within six years, compared to people who accepted medically proven therapies, researchers found.

“In our clinical practice, we started seeing patients coming in with more advanced cancer . . . because they first tried alternative therapies that failed,” said lead author Dr. Skyler Johnson, of the Yale School of Medicine and the Yale Cancer Center in New Haven, Connecticut.

Many cancer patients add nonmedical therapies to the treatments prescribed by their oncologists. But little is known about patients who choose only unconventional methods to address their cancer, Johnson and colleagues write in the Journal of the National Cancer Institute, online August 10.

To find out more about this group of patients, they used information collected on prostate, breast, lung and colon cancers for the U.S. National Cancer Database between 2004 and 2013.

The researchers had data on 280 people who tried only unproven methods administered by nonmedical personnel. They compared each of these patients to two people with similar cancer type, disease stage, age, race and other attributes, but who received conventional treatments.

Half of the patients were followed for at least five and a half years.

Compared to patients who chose evidence-based cancer treatments, those who used unconventional methods tended to have high social and economic status, be from northwestern U.S. states, have advanced cancers and be in otherwise good health.

Overall, patients who chose unproven methods were more than twice as likely to die during the follow-up period than those who received treatments like chemotherapy, radiation and surgery.

Compared to those receiving evidence-backed treatments, patients with breast cancer who opted for unproven methods were more than five times as likely to die, those with lung cancer were more than twice as likely to die and those with colon cancer were about five times as likely to die.

“Our findings highlight the importance of timely proven care for cancer,” Johnson told Reuters Health.

If the patients were followed for a longer period of time, it’s possible the differences could be greater, he said. Some prostate and breast cancers develop slowly, even if untreated, and may not be deadly within five to six years.

In addition, Johnson said, the researchers couldn’t account for people who received science-based treatments when their unconventional methods failed.

He said people should be cautious about what treatment advice they receive from the Internet or through word of mouth.

“This is something they need to think a lot about, because choosing alternative medicine for their cancer treatment could risk their lives,” Johnson said.