Over-the-counter antihistamines are a blessing for the more than 50 million Americans who suffer from allergies. Allergies are the sixth leading cause of chronic illness in the United States, according to the Asthma and Allergy Foundation.

But it's important to be cautious when taking these medications, as they could have serious side effects for some people, especially the elderly. In fact, a study published in the JAMA Network revealed that antihistamines may even increase your risk for Alzheimer's disease and dementia.

"People think that over-the-counter medications which don't require a prescription are safe, but all drugs have side effects," Dr. Gabe Mirkin, M.D., a noted physician from Orlando who is board-certified in Allergy and Immunology, tells Newsmax.

Dr. Andy Nish M.D., a top allergy specialist from Gainesville, Georgia adds:

"The older generation antihistamines such as Benadryl, chlortimeton, and hydroxyzine cross the blood brain barrier and cause sedation and also what we call anticholinergic side effects such as dryness and constipation.

"The newer 'nonsedating' antihistamines cross the blood brain barrier minimally or not at all," he tells Newsmax. "Zyrtec causes about 14% sedation and the others around 6%. Some people can be particularly prone to side effects while others less so, particularly with the newer antihistamines."

Here are some other side effects:

• Drowsiness and fatigue. Mirkin says medications that block histamine production are helpful for relieving nasal stuffiness but can cause drowsiness. "They all cause fatigue," he says, "and should be taken with caution, especially by older folks."
• Dry mouth. According to Self, dryness is a common side effect of antihistamines, especially the first generation of drugs such as Benadryl.
• Dry eyes. Experts told Self that antihistamines can reduce tear production and your eyes may become red and irritated and may even sting.
• Constipation and urinary retention. These drugs have an effect on how the neurotransmitter acetylcholine acts in the body. When the movement of acetylcholine slows down because of the medication, so will your gut muscles. The same mechanism can affect the muscles of the bladder, making it difficult to eliminate totally.