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A drug used to treat rheumatoid arthritis may have benefits against Alzheimer’s disease, researchers report.

Rheumatoid arthritis is an autoimmune disease believed to be driven in part by tumor necrosis factor, or T.N.F., a protein that promotes inflammation. Drugs that block T.N.F., including an injectable drug called etanercept, have been used to treat rheumatoid arthritis for many years.

T.N.F. is also elevated in the cerebrospinal fluid of Alzheimer’s patients.

Researchers identified 41,109 men and women with a diagnosis of rheumatoid arthritis and 325 with both rheumatoid arthritis and Alzheimer’s disease. In people over 65, the prevalence of Alzheimer’s disease was more than twice as high in people with rheumatoid arthritis as in those without it. The study is in CNS Drugs.

But unlike patients treated with five other rheumatoid arthritis drugs, those who had been treated with etanercept showed a significantly reduced risk for Alzheimer’s disease.

Still, the lead author, Dr. Richard C. Chou, an assistant professor of medicine at Dartmouth, said that it is too early to think of using etanercept as a treatment for Alzheimer’s.

“We’ve identified a process in the brain, and if you can control this process with etanercept, you may be able to control Alzheimer’s,” he said. “But we need clinical trials to prove and confirm it.”