Results from a large U.S. government experiment are dimming hopes that two common painkillers can prevent Alzheimer's disease or slow mental decline in older people.

The arthritis drug Celebrex and the painkiller Aleve showed no benefit for thinking skills, new findings suggest.

Aleve is a low-strength version of the prescription drug naproxen sodium, known under the trade names Anaprox and Naprelan, that is sold in the U.S. as an over-the-counter drug.

Celebrex is the trade name for the prescription drug celecoxib, known as a cox-2 inhibitor.

Earlier results from the same research showed the two drugs didn't prevent Alzheimer's, at least in the short term.

The experiment was halted in 2004, several years before the intended end date, when heart risks turned up in a separate study on Celebrex because of an increased risk of cardiovascular events. (Another cox-2 inhibitor, Vioxx, was pulled from the market entirely because of similar heart risks that same year.) Researchers also had noticed more heart attacks and strokes in the people taking Aleve in the Alzheimer's prevention study.

Despite the study's early end, there was still enough data to hint at how the drugs affect thinking and memory. The findings were posted online Monday and will appear in July's Archives of Neurology.

"These were not the results we were hoping for," said co-author Barbara Martin of the Johns Hopkins Bloomberg School of Public Health. "We designed this study hoping we would see a protective effect of these drugs."

Researchers hope to continue monitoring the participants to see whether they exhibit any delayed benefit.

Researchers thought drugs might alleviate brain inflammation

Scientists have speculated that nonsteroidal anti-inflammatories, such as naproxen sodium (or naproxen on its own, sold as Naprosyn) and the cox-2 inhibitors, might prevent Alzheimer's by reducing inflammation in the brain or by other means.

"The drugs have several effects in the brain, and the different effects could be important at different stages in the illness," said study co-author Dr. John Breitner of the University of Washington in Seattle.

Previous studies had found that people who took the drugs ran a lower risk of developing Alzheimer's. But those were observational studies, meaning they observed people's behaviour and health. The people who took the pills may have had other healthy habits that lowered their risk.

The halted study included more than 2,000 people age 70 and older with a family history of Alzheimer's but no thinking problems themselves. People were randomly assigned to take standard daily doses of either Celebrex, Aleve or a dummy pill.

At the start and annually for up to three years, they took a battery of tests. In one, they named as many grocery items as they could in one minute.

All three groups scored about the same at the start. But over time, those taking Aleve scored on average slightly lower than the people who took placebos. Those who took Celebrex scored slightly lower than those on placebo on most, but not all, of the tests.

"There's no evidence that people should be on these drugs to prevent Alzheimer's disease," said Dr. David Bennett of Chicago's Rush University Medical Center, who was not involved in the study but does similar research. "With the side effects of these drugs, people shouldn't be taking them for this reason."

Both products now carry warnings about heart risks. Anti-inflammatory drugs also can cause serious gastrointestinal bleeding. Experts advise patients to ask their doctors about how long to take the drugs for pain.

The study was funded by the National Institute on Aging. Pfizer Inc. and Bayer Healthcare provided the drugs and matching dummy pills for the study but did not participate in the research. Some of the authors reported receiving speaking or consulting fees from drug companies, including Pfizer.

"Unfortunately, as this paper demonstrates, research doesn't always translate into successful new treatments," said Dr. Gail Cawkwell of Pfizer in an e-mail. Cawkwell pointed out that the study did not find increased heart risks for Celebrex.