A nurse is caring for a client in active labor. The nurse identifies tachysystole and late decelerations with each contraction. The nurse performs intrauterine resuscitation interventions, including repositioning the client, initiating an IV fluid bolus, and applying oxygen. After the stated interventions, the nurse continues to observe tachysystole and recurrent late decelerations. Which medication does the nurse anticipate administering to reduce uterine activity?