Updated March 16, 2021
Allergy season can bring a whole host of unpleasant symptoms, like a runny nose, sneezing and itchy, watery eyes. An allergic reaction is a chain reaction that starts in your genes and is expressed by your immune system, according to the American Academy of Allergy Asthma and Immunology. Certain allergy medications, including first-generation antihistamines like diphenhydramine (Benadryl) can make you feel tired, says Stanley A. Schwartz, MD, PhD, UB Distinguished Professor of medicine and pediatrics, who is chief of the Division of Allergy, Immunology and Rheumatology. “Antihistamines block the receptors for histamine in the body, and there is a receptor for histamine in the brain that keeps you alert,” he explains.