Gastrointestinal Side Effects of Antihistamines

Evidence-Based Comparison

The American Gastroenterological Association notes that diphenhydramine causes epigastric discomfort as a recognized adverse effect when used for sedation and allergic conditions 1, 2
The anticholinergic properties of diphenhydramine contribute to dry mouth, constipation, and gastrointestinal symptoms, according to the American Academy of Allergy, Asthma, and Immunology 3, 4
These GI effects are part of the broader anticholinergic syndrome associated with first-generation antihistamines, as reported by the American College of Gastroenterology 1, 2

Diphenhydramine has stronger anticholinergic effects that directly impact GI motility and secretion, leading to epigastric discomfort, dry mouth, and constipation, according to the American Gastroenterological Association 1, 2, 4
Hydroxyzine has additional antiserotonergic properties that may actually counteract some GI symptoms, particularly in conditions with serotonin-mediated GI distress, as noted by the American Academy of Allergy, Asthma, and Immunology 5

The American Geriatrics Society recommends that both medications be avoided in elderly patients due to cognitive effects 3, 5
For patients with pre-existing GI sensitivity or complaints, hydroxyzine would be the preferred first-generation antihistamine if one must be used, according to the American College of Allergy, Asthma, and Immunology 5
In mast cell disorders with GI symptoms, hydroxyzine's antiserotonergic properties may provide additional benefit beyond antihistamine effects, as reported by the American Academy of Allergy, Asthma, and Immunology 5