Moderate
Cetirizine × Morphine
Second-generation antihistamines×Natural opium alkaloids
Mechanism
Cetirizine penetrates the blood-brain barrier at higher doses. With opioids, additive CNS depression and sedation occur.
Symptoms
Sedation, fatigue, slowed reactions, impaired coordination. In older patients: fall and fracture risk. Driving and operating machinery: accident risk.
Management
On the combination, cap cetirizine at 10 mg/day (at bedtime). Avoid the combination in older patients — fall risk. In palliative care, alternatives: loratadine (minimal sedation) or bilastine (minimal sedation and no sedating metabolites). With marked sedation, stop the antihistamine.