174 adult patients from the orthopedic surgeons undergoing total knee replacement with spinal anesthesia will be enrolled. The study will be double blind, randomized; placebo controlled comparing oral and IV acetaminophen to each other and a control (placebo). At the time patients arrive in the post-anesthesia care unit (PACU), Group 1 will receive IV 1 gram acetaminophen, and PO placebo; Group 2 will receive PO 1 gram acetaminophen and IV placebo; Group 3 will receive IV placebo and PO placebo.