NEW YORK (Reuters Health) - Older adults with complex medication regimens may make potentially harmful mistakes in taking their drugs as prescribed, a new study shows.

They are particularly prone to these errors when their medication regimen has been modified, or if they are taking multiple drugs, Dr. Terry S. Field of the Meyers Primary Care Institute in Worcester, Massachusetts, and colleagues found.

The findings show how important it is for patients to understand exactly how they are supposed to be taking their prescription medications, and for doctors to make sure their patients are getting the message, Field told Reuters Health. "It's very easy to walk out the door and really not be certain what it is that you've heard."

To date, most research on patients and their medication has focused on adherence to taking drugs as prescribed, Field said. She and her colleagues decided to look at the nature and impact of errors patients made in actually taking the drugs. "That turned out to be a much more important part of the problem than I think anybody had been recognizing."

She and her colleagues followed 30,000 Medicare patients for a year and identified 129 adverse drug events, or potential adverse drug events, due to patient errors.

In 42 percent of the cases, errors were made after doctors had asked patients to modify their medication regimen.

Furthermore, those taking three to four drugs were twice as likely to make mistakes as those on two or less, while patients taking more than seven medications were more than three times as likely to make errors.

Dementia, confusion and sensory problems were the most common underlying cause for patient medication errors, Field and her team found.

"Whenever patients develop medical or health conditions that weaken their ability to cope with medication regimens, a fresh review of the risks and benefits of their medications may be required," they advise in their report in the Journal of the American Geriatrics Society.

Field noted, for example, that some patients who are prescribed an old medication at a new dose might keep taking the old dosage along with the new dosage. In other instances, they fail to follow certain instructions; for example, to take a bowel preparation along with pain medications in order to prevent constipation.

She advises patients to bring in their medications when meeting with their doctor to make sure they are taking them properly, and not to be afraid to ask questions if they don't understand. "It's very easy to be come very passive in that setting."