Researchers at Families USA found insurers participating in the Medicare drug benefit program raised overall prices 3.7 percent for the top 20 drugs used by the elderly since enrollment began in November.
Separately, the AARP found prices for nearly 200 of all drugs most used by the elderly -- not just under Medicare -- rose 3.9 percent from January through March. The AARP is the largest group representing older Americans.
The drug benefits program, which began in January, allows insurers to offer Medicare beneficiaries drug coverage with government oversight.
Participants are locked into a plan until annual open enrollment, although plans are allowed to change prices any time. Many patients were automatically transferred into the program, but about 11 million signed up individually.
"At the same time that the Bush administration and congressional leaders are touting the effectiveness of the Medicare drug plans, those plans are quietly raising the prices that they charge," said Ron Pollack, director of Families USA.
The Centers for Medicare and Medicaid Services and an insurance industry group had no immediate comment on the studies.
The Pharmaceutical Research and Manufacturers of America, disputed the AARP study, saying data shows drug prices are not rising any more than other medical costs.
Medicare officials have said competition between companies offering the drug benefit would help lower co-payments and annual fees.
According to Families USA, prices for three Pfizer Inc drugs rose at least 6 percent -- painkiller Celebrex, cholesterol drug Lipitor and Alzheimer's disease therapy Aricept.
Drugs in the study that did not have price hikes included the diuretic generic drug furosemide, generic metoprolol tartrate for high blood pressure, and Pfizer's antidepressant Zoloft, which could soon face generic competition.