Common allergy and cold medicines can increase the risks of developing Alzheimer's disease, a study found last year. Now, a recent study found the cause: these common drugs can shrink the brain.

The 2016 study found that patients who regularly took the common drugs to treat colds and allergies in an average of 2.5 years had smaller brain sizes.

Researchers from the Indiana University School of Medicine analyzed the brain scans of patients and tested their memory and cognitive aptitude in a series of tests.

The study found that these people had compromised brain metabolism. Moreover, they had poor memory and cognitive test scores compared to folks who did not take the common drugs in the same time span.

Collectively, these common allergy and cold medicines are called "anticholinergics." They block a chemical called "acetylcholine," which plays a role in the nerve cells' transmission of electrical impulses.

Alzheimer's patients lack acetylcholine. Experts are concerned that these common drugs may trigger and worsen the disease.

"These findings provide us with a much better understanding of how this class of drugs may act upon the brain in ways that might raise the risk of cognitive impairment and dementia," said the study's first author Dr. Shannon Risacher, an imaging sciences and radiology assistant professor.

The current research enrolled 451 patients drawn from two other studies - the Indiana Memory and Aging Study and the nationwide Alzheimer's Disease Neuroimaging Initiative. Sixty of the patients were regularly taking at least one of the drugs that have medium to high anticholinergic activity.

Apart from the cognitive and memory tests given, the research team used magnetic resonance imaging (MRI) to study the brain structure and positron emission tests (PET) to measure the brain's metabolism.

Risacher added that the findings can provide other researchers with clues on how to better understand the biological foundation of cognitive issues linked to anticholinergic drugs. It can also guide other teams to further study the link between the common drugs and Alzheimer's disease.

The research evidence can also aid doctors in providing a better alternative to these medicines for senior patients. The study was published in the JAMA Neurology journal on April 18.

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