Pretreatment with antihistamines for the prevention of hypersensitivity infusion reactions is recommended for certain biologics and chemotherapies. Cetirizine is the first injectable second-generation antihistamine recently approved for acute urticaria. A randomized, exploratory phase 2 study evaluated intravenous (IV) cetirizine 10 mg versus IV diphenhydramine 50 mg as pretreatment in patients receiving an anti-CD20 agent or paclitaxel. In the overall population (N = 34) and an elderly subgroup (n = 21), IV cetirizine was as effective as IV diphenhydramine in preventing infusion reactions (primary outcome) and associated with less sedation at all time points, a shorter infusion center stay, and fewer treatment-related adverse events.