BIG PHARMA SWEETHEART –
July 16, 2025 – Benadryl causes drowsiness by blocking H1 receptors and crossing the blood-brain barrier
But it also does other things that can create side effects. For instance, it blocks sodium channels, helping it to act as a local anesthetic and potentially leading to dangerous cardiac effects. Additionally, it can cause dry mouth, blurry vision, constipation, urinary retention, and an elevated heart rate because it blocks acetylcholine, a neurotransmitter in the central nervous system, Trisha Ray, MD, clinical director of Allergy and Immunology at Beth Israel Deaconess Medical Center, told Health.
Due to these significant side effects, manufacturers eventually developed “second-generation” antihistamines, such as loratadine (Claritin), cetirizine (Zyrtec), or fexofenadine (Allegra), to offer gentler options for the body.4
These newer medications bind only to H1 receptors and cross the blood-brain barrier at much lower levels than Benadryl, making them less likely to cause drowsiness and other adverse effects, said Ray.
