Individuals with allergies often rely on antihistamines to alleviate their symptoms. However, these drugs may increase or even cause symptoms of dementia, according to a study in the journal JAMA Internal Medicine.

Antihistames have “anticholinergic” effects which means they can block the action of a natural substance called acetylcholine. This substance transmits messages in the nervous system. In the brain, it is involved in learning and memory; in the rest of the body, it stimulates muscles to contract.

Antihistamines are one of a number of drugs with anticholinergic effects. Others include tricyclic antidepressants, some muscle relaxants, some intestinal relaxants, treatments for dizziness, and drugs that treat overactive bladder.

Most people, including doctors, know the common side effects of anticholinergic drugs which include sleepiness, confusion, and problems with short-term memory and reasoning. However, many are just beginning to realize that these drugs can increase and even cause dementia.

Researchers studied about 3,500 men and women, ages 65 and older, over the course of seven years and tracked participants’ use of both prescription and over-the counter (OTC) medications, both prior and during the study. They also tracked their health.

During that period, 800 developed dementia. Researchers found that those people who used drugs with anticholinergic effects were more likely to develop dementia and had greater risk if they took higher doses over longer periods of time.

Those in the study who took the highest doses for the longest time had a 54 percent higher dementia risk than individuals who took lower doses for less time. This study reaffirmed what earlier studies had concluded but was the first to include both prescription and OTC medications.

I stopped taking antihistamines several years ago because of side effects. I find this study upsetting because other members of my family and people I know regularly take OTC antihistamines with anticholinergic effects.

Drugs with anticholinergic effects are widely used and even prescribed to half of all dementia patients. OTC Benadryl is one listed with strong negative side effects.

Avoiding all anticholinergics is not always possible but awareness of the problem can help you decide if the benefits outweigh the risks. Ask your doctor about alternative antihistamines if you are taking one with anticholinergic effects.