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Why can antihistamines cause dry mouth and blurred vision?
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antihistamine
Antihistamines such as Benadryl (diphenhydramine) and Claritin (loratadine) can be purchased without prescriptions. Both of those agents act by blocking H1 receptors. However, whereas Benadryl binds to those receptors in the central nervous system (CNS), causing drowsiness, Claritin does not readily enter the CNS and thus does not normally cause drowsiness.
Antihistamines can cause side effects such as dry mouth and blurred vision because many of these drugs, particularly older H1 receptor antagonists, also possess anticholinergic properties.12
Anticholinergic agents work by blocking the physiological action of acetylcholine, a neurotransmitter that regulates various bodily functions. When an antihistamine interferes with these receptors, it disrupts normal autonomic processes, leading to common adverse effects like dryness of the mouth and blurred vision. While these side effects are well-documented, the primary function of antihistamines remains the selective counteraction of histamine to alleviate symptoms of allergy and other conditions.12