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You are seeing a 6 year old with 1 ½ weeks of cough, nasal congestion, and malaise. Initially, he seemed to be improving a bit, but now for the last 2 days he has a fever and worsened cough, post-tussive and spontaneous vomiting, as well as a new sore throat and conjunctival injection. His vital signs are: temperature is 38.2, HR 110, RR 32, and pulse ox 96% on room air. On exam he is nontoxic but has some crackles heard bilaterally. Your plan is to treat him with azithromycin for community-acquired pneumonia, pseudoephedrine during the day and diphenhydramine at night for his nasal congestion, and ondansetron for his vomiting. 

The pharmacist calls you to discuss potential interactions between which 2 medications you prescribed?