Alcohol and other CNS depressants (benzodiazepines, opioids, sleep medicines) increase sedation, other antihistamines increase side effects, anticholinergics increase dry mouth/constipation/urinary retention, QT-prolonging drugs (e.g., certain antiarrhythmics, antipsychotics, macrolide antibiotics, fluoroquinolones) increase arrhythmia risk, MAO inhibitors may enhance anticholinergic/CNS effects