A drug commonly prescribed to combat depression has turned out to have an unexpected side effect that could possibly have an impact on the treatment Alzheimer's disease, researchers say.

The antidepressant citalopram, better known under its brand name Celexa, was found to decrease the creation of a protein that in brains of Alzheimer's sufferers binds together as sticky, hard plaques believed to short-circuit wiring in the brain.

Previous studies have implicated such plaques in the memory losses and other mental cognitive impairments associated with Alzheimer's.

In a study with mice that have been bred to develop Alzheimer's and with healthy human volunteers, citalopram lowered the levels of the suspect protein, known as beta-amyloid, found in cerebrospinal fluid by 38 percent.

Although the cerebrospinal fluid is outside the brain, the researchers say the findings are a clear indication the protein would be at lower levels in the brains of people using the antidepressant.

However, researchers have urged caution in considering it a possible way to slow or arrest Alzheimer's.

"Antidepressants appear to be significantly reducing amyloid beta production, and that's exciting," said senior study author Dr. John Cirrito, a neurology professor at Washington University School of Medicine. "But while antidepressants generally are well tolerated, they have risks and side effects. Until we can more definitively prove that these drugs help slow or stop Alzheimer's in humans, the risks aren't worth it. There is still much more work to do."

Amyloid beta is a byproduct of normal activity in the brain, and it is only when levels increase and it binds together to create plaques that it is implicated in Alzheimer's, the researchers point out.

It remains to be seen -- and researched -- whether reducing levels of beta-amyloid in the brain can alter the progression of Alzheimer's, some experts say.

Also, dosage levels in the mice/human study, around 60 milligrams, are three times as high as recommended for people age 60 and above, they warn.

"People should not start popping citalopram or other antidepressants in the expectation that they will prevent Alzheimer's disease," says Dr. Lon Schneider, an Alzheimer's specialist at USC, who was not involved in that study. "They could be doing some substantial harm."