Drug Interactions Between Diphenhydramine and Loratadine

Mechanism and Interaction Potential

• The American College of Allergy, Asthma, and Immunology recommends considering the potential for additive anticholinergic effects when combining diphenhydramine with other medications, although loratadine itself has minimal anticholinergic properties 3, 2

Clinical Performance and Safety Concerns

• The American Geriatrics Society identifies diphenhydramine as an inappropriate medication that induces delirium postoperatively in older adults, and recommends avoiding its use in this population due to increased risk of delirium, urinary retention, dry mouth, blurred vision, constipation, and impaired judgment 3
• Diphenhydramine should be avoided in patients with bladder neck obstruction, glaucoma, or existing urinary difficulties due to its anticholinergic effects, as stated by the American Academy of Ophthalmology and the American Urological Association 1, 4

Clinical Recommendations

• The American Academy of Allergy, Asthma, and Immunology recommends that there is no therapeutic rationale for combining diphenhydramine and loratadine, as both are H1-antihistamines treating the same symptoms through the same mechanism 1
• Second-generation antihistamines like loratadine are generally preferred over diphenhydramine for allergic rhinitis treatment due to superior safety profile, as recommended by the American College of Allergy, Asthma, and Immunology 1
• The American Geriatrics Society recommends avoiding diphenhydramine in older adults, young children, and patients with cardiac arrhythmia, hypertension, bladder neck obstruction, glaucoma, or hyperthyroidism due to its potential for adverse effects 1, 3