The less medicine they take, the less likely the elderly were to enroll in a new Medicare drug plan, a signal that many senior citizens still don't view the program as insurance in case their health deteriorates.

Researchers say the trend was particularly noticeable among the poor and those without a college education. About 40 percent of these seniors did not have drug coverage — and were not taking medication.

"We speculate that this arises because of constraints or perceptions that make it difficult for people in these groups to account for future benefits," said the authors of an article, published Tuesday in the journal Health Affairs.

The authors concluded that healthy people with low education should be targeted with counseling on the future benefits of coverage, and that waiving a penalty for late enrollment could help bring more into the program.

As incomes rose, the elderly were more likely to have drug insurance, even if they were taking no medications. For those with more than $20,000 in income annually, the percentage without drug coverage dropped to 11.3 percent.

Seniors taking three or more medications were far more likely to have drug coverage, even among the poor. Only 5.2 percent of these people did not have prescription drug coverage, the authors wrote.

The researchers based their conclusions on a survey of 2,137 seniors who were interviewed just before enrollment into the program began and again once enrollment ended.

"The Part D program has performed better than some people feared, but it has also left a significant chunk of people uncovered," said Daniel McFadden, a lead study author and a professor of economics at the University of California, Berkeley. "Many are people that any system would have difficulty reaching and helping make informed choices."

Other authors were Florian Heiss and Joachim Winter of the University of Munich in Germany. The study was funded through the National Institute on Aging and the E. Morris Cox Fund at the University of California at Berkeley.

Seniors seemed conflicted in how they view the program. While 57 percent of respondents described the program as a major benefit, only 30 percent said it was well designed.

"When Medicare was passed in 1965, it was also hotly contested and debated," said Leslie Norwalk, deputy administrator for the Centers for Medicare and Medicaid Services. "Over time, people have grown to love their Medicare program, and I suspect people will grow to love their drug benefit. It just may take a while for the political waters to calm."

Judith Stein, executive director of the Center for Medicare Advocacy Inc., an advocacy group, said seniors don't like the design of the program because it leads to confusion rather than maximum value.

"Part D should be revisited and redesigned to offer a standardized prescription drug program through the traditional Medicare program," Stein said. "Such a program would be more valuable to more people with Medicare and less expensive for taxpayers."