WASHINGTON (AP) -- Researchers say that when many people who enrolled in Medicare hit what's called the "doughnut hole," they stop taking their medicines.

About 3.4 million older and disabled people dealing with diabetes, high blood pressure and other chronic conditions hit that gap last year.

When that happens, they have to pay the entire costs of their medicine until they spend $3,850 out of pocket. After that, insurance kicks in again.

The study looked at eight categories of drugs. On average, 15 percent of those hitting the coverage gap stopped their treatment regimen, though that varied depending on the illness.

The lowest rate was for Alzheimer's patients. Only 8 percent ofthose stopped taking their drugs. Those who take medicine for heartburn, ulcers and acid reflux disease quit at the highest rate, 20 percent.

The Kaiser Family Foundation conducted the study with researchers at Georgetown University and the University of Chicago.

(Copyright 2008 by The Associated Press. All Rights Reserved.)