WASHINGTON (Cox News Service) -- The Medicare prescription drug law significantly expanded drug coverage for the elderly, but left many with higher out-of-pocket costs than people covered by employers or the Department of Veterans Affairs, health researchers reported Tuesday.

The researchers found that millions of low-income Medicare beneficiaries who qualified for drug subsidy payments were not receiving them.

A study of about 16,000 beneficiaries conducted last October found that the percentage of elderly people without prescription drug coverage fell to 8 percent last year, from 33 percent before the Part D drug benefit took effect in 2005.

But the study also found that one in four Medicare beneficiaries enrolled in a Part D prescription drug plan reported spending at least $100 a month in out-of-pocket prescription drug costs, and 8 percent reported spending at least $300 a month.

By comparison, less than 5 percent of elderly beneficiaries who were covered either by an employer plan or a VA plan spent at least $300 a month. The survey did not determine what percentage of those beneficiaries spent more than $100 a month.

The study was sponsored by the Kaiser Family Foundation, the Commonwealth Fund and Tufts-New England Medical Center. It had a margin of error of less than 1 percentage point. The study was published in Tuesday's edition of the policy journal Health Affairs.

"The Medicare drug law achieved its primary goal of providing drug coverage to most seniors who previously lacked it," Drew Altman, the president and chief executive officer of the nonprofit Kaiser Family Foundation said in a statement.

"But the survey found a significant number of seniors in Part D plans paying sizeable amounts out of pocket for their medications," Altman said.

The study found that 20 percent of Part D beneficiaries reported they had delayed filling or not filled a prescription in the past year because of cost reasons. By comparison, 8 percent of beneficiaries covered through an employer plan and 12 percent covered by the VA reported skipping medication for cost reasons.

The elderly who still did not have any drug coverage after the law took effect fell primarily into two groups: those who considered themselves healthy enough not to need insurance and so-called "vulnerable" beneficiaries. Those include people over 75, minorities, those with low education levels, and those with incomes 150 percent or less of the federal poverty level, or about $15,300.

Despite aggressive outreach and enrollment efforts by Medicare and Social Security officials, about half of those surveyed with low incomes said they were not aware of the benefit. An estimated 3.4 million to 4.7 million beneficiaries who are eligible for low-income subsidies do not receive them, according to the report.

"Low-income subsidies really make a difference for those seniors who receive them, but there are still millions of low-income seniors who are eligible for this help but are not getting it," said Tricia Neuman, a Kaiser vice president and director of the foundation's Medicare Policy Project and lead author of the study. "Our study confirms the importance of doing more to get additional assistance to low-income seniors."

MEDICARE DRUG SUBSIDY: ARE YOU ELIGIBLE?

To qualify for a low-income Medicare Part D subsidy, you must:

-- Have Medicare Part A (hospital insurance) or Medicare Part B (medical insurance)

-- Live in one of the 50 states or the District of Columbia

-- Have combined savings, investments, and real estate (other than your home) of less than $11,710 for an individual or $23,410 for a married couple.

How to apply for the subsidy:

-- Phone the Social Security Administration toll-free at 1-800-772-1213

-- Find an application at Social Security's Web site, www.ssa.gov/prescriptionhelp/

-- Visit any Social Security Administration office.

Source: Social Security Administration

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