A 32-year-old woman with community-acquired pneumonia caused by Streptococcus pneumoniae is prescribed azithromycin 500 mg daily. On day 3 of therapy, she develops severe nausea, vomiting, and abdominal cramping. Stool culture is negative for Clostridioides difficile. Her renal and hepatic function are normal. What is the most appropriate next step in management?