Washington ? Nearly half of all new prescription drugs introduced between 1989 and 2000 may not be much better than existing medications, according to a report by a leading health-care research group. The same drugs accounted for nearly a quarter of the growth in retail prescription drug spending in recent years, the study found.

The drugs in question most of which are upgrades of older medications are protected by patents from competition for three years.

The effect is to reduce patients’ access to cheaper generic drugs that can be equally effective. If these marketing trends continue, they could mean that patients get less effective drug treatments while spending more for them.

The practice of re-patenting older drugs is likely to accelerate as 17 branded drugs with U.S. sales of almost $20 billion lose their patent protection over the next four years. With few innovative breakthrough drugs in the developmental pipeline, experts say the drug industry will lean heavily on re-patenting older drugs to help keep profits and prices high.

According to Tuesday’s study, only 35 percent of drugs approved by the Food and Drug Administration between 1989 and 2000 were based on new chemicals acting in new ways to treat a disease. Most of the remainder were modifications of drugs whose active ingredients had already been approved.

“The plain fact is that many new drugs are altered or slightly changed versions of existing drugs and they may or may not be all that much better than what’s already available. Consumers should be more aware of that,” said Nancy Chockley, president of the National Institute for Health Care Management Foundation, the Washington-based non-profit research group that released the report. NIHCM was founded by Blue Cross-Blue Shield and takes a health care manager’s point of view when it comes to drug prices.

“New” old drugs, including big sellers such as Clarinex, Nexium and Glucovance, are made by tweaking the existing drugs’ formulas, dosages and other properties to win renewed patent protection. They then are marketed aggressively to doctors and the public through advertising.

Drug industry representatives called the new report a “politically and financially-motivated cheap shot,” noting that 11 of the institute’s 12 board members are Blue Cross-Blue Shield executives who are critical of drug industry pricing.

“To say that drugs which lengthen the lives of AIDS patients, reduce the ravaging physical torment of cancer treatments and dramatically reduce the number of major heart surgeries and hospitalizations is not innovation is to insult the millions of Americans whose lives have been made better by the invention of these drugs,” said Richard Smith, vice president of the Pharmaceutical Research and Manufacturers of America, a Virginia-based trade group representing the drug industry.

Michie Hunt, a Washington-based consultant and author of the report, acknowledged that the industry had produced innovative new drugs to treat cancer, AIDS, erectile dysfunction and other maladies. They were not included among drugs considered to have provided little benefit over existing drugs.

Separately, AARP, the huge seniors lobbying group, will announce today that it is joining in a number of those lawsuits against brand-name drug companies that try to delay the marketing of low-cost generic medicines by “late filing” patent applications to thwart generic competitors. By law, these disputes are allowed at least 30 months of argument, providing the brand-name maker with 2 1/2 of protection from generic competition.