CHICAGO (Reuters) - Most doctors rely on patients to give them an accurate account of what drugs they are taking, but a new U.S. study published on Thursday suggests many patients get it wrong.

About 40 percent of 119 patients taking blood pressure medication in three community health centers could not accurately recall what drugs they were taking.

That number jumped to 60 percent for those with low health literacy, a measure of their ability to read and comprehend health-related materials, researchers at Northwestern University's Feinberg School of Medicine in Chicago found.

This could lead to drug interactions and the undertreatment of chronic diseases, said Dr. Stephen Persell, whose study will appear in the November issue of the Journal of General Internal Medicine.

"I don't think we have a good grasp on how important this problem is in terms of the medical outcomes," he said in a telephone interview.

Persell said the problem was worse than expected and poses challenges for doctors who are trying to understand why a patient's health is not improving with treatment.

"Does it mean the patients are not responding well to the medication or are they not using the medication?" he said.

Persell and colleagues studied 119 patients with high blood pressure and an average age of 55 at three community health centers in Grand Rapids, Michigan.

People were asked to name their blood pressure medications and the researchers compared their answers to the drugs listed in their medical charts.

They found about one-third of patients with adequate health literacy were unable to name their drugs accurately.

Even a look at a patient's medical records may not present an accurate picture of the drugs a patient is actually taking, he said.

Some of his own patients have continued taking prescription drugs even though he prescribed a different drug and told them to stop.

"Patients and doctors have to be in agreement about what drugs patients are actually taking," he said.

One possible solution is for patients to physically bring their medicines to the doctor's office.

"If they have to go to the hospital, they should bring their pill bottles with them. They should know the name of their pharmacy, so if there is a question, the healthcare workers can call the pharmacy," he said.

He also suggested that drugmakers should be encouraged to simplify the names of drugs once they become generic. Currently, generic drugs are sold by their chemical names, which are unpronounceable for the average patient.

"We need a system-wide approach for this," Persell said.