The extent to which pain reduction is observed with an analgesic in a RCT may depend on the surgical procedure studied because if the procedure intrinsically is not very painful, then it will be more difficult to demonstrate a pain relief benefit. In the available pub-lished RCTS,IV acetaminophen was studied in surgeries associated with varying levels of pain, including Cesareans, total abdominal hysterectomy, tonsillectomy, coronary artery bypass grafting, thyroidectomy, hip or knee replacement, and laparoscopic cholecystectomy procedures. The average postoperative rest pain scores in the placebo arms of these studies ranged from 2.0 to 3.7 out of 10.
and for dynamic pain, it ranged from 3 to 5 out of 10. Two of the 14 placebo-controlled studies found no pain improvement or less opioid use with IV acetaminophen. One study involved patients undergoing lumbar discectomy who were administered a single 1-g dose 45 minutes before the end of surgery and with pain followed for 2 hours in the post-anesthesia care unit (PACU) (reference 6 in Table 1). This study also found no difference between IV acetaminophen and an active comparator IV parecoxib, whereas IV metamizol patients had lower pain scores on arrival to PACU with fewer patients requiring rescue.