BACKGROUND: A patient was treated with abemaciclib and rivaroxaban. Therapeutic drug monitoring revealed high total plasma concentrations of rivaroxaban, which was identified as likely cause of abnormal bleeding. This result indicating that it was caused by drug-drug interaction. METHODS: According to the pharmacokinetic profile, the concentrations of rivaroxaban, abemaciclib, and its metabolite (M2) were monitored. RESULTS: The concentrations of abemaciclib and its metabolite (M2) decreased with the reduction in rivaroxaban dosage. CONCLUSIONS: This case study demonstrates the interactions between abemaciclib and rivaroxaban. For patients with complex medications, therapeutic drug monitoring can reduce the occurrence of adverse effects.