Plenty of evidence suggests that having trouble understanding medical information is bad for your health. Now new research says it could even be deadly.

A study of patients 65 and older found that those who couldn't understand basic written medical instructions were much more likely to die within six years than those who had no problems grasping the information.

The difference in the death rates remained substantial even when researchers considered differences in the patients' health at the outset.

Inability to understand medical information and instructions makes it hard to manage chronic illnesses from asthma to diabetes to heart disease, said lead author Dr. David Baker, chief of general internal medicine at Northwestern University's Feinberg School of Medicine.

That in turn can lead to declining health, frequent hospitalizations and ultimately death, especially in older patients whose health may be more precarious to begin with, he said.

One-fourth of the 3,260 patients in the study were considered medically illiterate. That was based on tests of their ability to read common medical information, including prescription labels, appointment slips and instructions on how to prepare for an X-ray.

Almost 40 percent of those deemed medically illiterate died during the study, compared with 19 percent of those who were literate. Factoring in health at the outset and other variables, medically illiterate patients were 50 percent more likely to die than the others.

The difference in death rates "was much higher than we expected," Baker said.

The results appear in Monday's Archives of Internal Medicine.

Evidence suggests that as many as 90 million Americans have trouble with medical literacy, said Dr. Joanne Schwartzberg, director of aging and community health at the American Medical Association. She was not involved in the study.

While patients of all ages are affected, "the elderly are the most highly challenged because they're on the most drugs and have the most chronic illnesses," she said.

Other studies have suggested poor literacy might be linked with higher death rates, but this is the most comprehensive to date, Schwartzberg said.

A recent small study suggested intensive education to help patients understand medical information can reduce hospitalization and death, she said.

The AMA has educational materials to help doctors improve literacy. Some methods are as simple as outlining instructions in three key points, or asking patients at the end of a doctor visit to repeat what they've just been told, Schwartzberg said.

Baker said cutting the medical jargon and explaining things both verbally and in writing also help.

Northwestern uses videos to boost patients' understanding. Offering medical information in different formats makes sense since people learn in different ways, he said.