Acute diarrhea following the first subcutaneous dose of tafolecimab: a case report of a probable adverse drug reaction

Background Tafolecimab is a fully human monoclonal antibody targeting proprotein convertase subtilisin/kexin type 9 (PCSK9). Diarrhea is a recognized adverse reaction, but detailed post-marketing case descriptions with clear timing, differential assessment, and outcome remain limited. Case Summary A 51-year-old man hospitalized for non-ST-segment elevation myocardial infarction received his first one-time 600-mg subcutaneous dose of tafolecimab (SINTBILO) at 15:14 on 13 July 2026. Approximately 7 h 46 min later, he developed abdominal pain and diarrhea occurring 5–8 times daily, with nausea and mild periumbilical pain but no fever, tenesmus, visible hematochezia or melena, dehydration, hypotension, rebound tenderness, or guarding. Stool testing showed loose yellow stool, positive occult blood and red blood cells, 0–1 white blood cells/high-power field, and a negative rotavirus test; electrolytes and liver and kidney function were normal. Stool culture and testing for Clostridioides difficile, norovirus, and adenovirus were not performed. Tafolecimab was not re-administered. After fluid replacement, montmorillonite, and loperamide hydrochloride, diarrhea decreased within 1 day and resolved after approximately 2 days, with no recurrence after discharge. Long-term stable acarbose therapy was continued without recurrence. The Naranjo score was 5, indicating a probable adverse drug reaction. Conclusion This case supports probable tafolecimab-associated acute diarrhea. New-onset diarrhea after PCSK9 inhibitor initiation should prompt precise temporal documentation, medication reconciliation, targeted microbiological evaluation, and pharmacovigilance reporting.