The purpose of the study is to examine the effects of timing of administration of intravenous (IV) acetaminophen and how timing relates to analgesic efficacy as measured by postoperative pain scores. A maximum of 138 patients meeting inclusion criteria who present for a scheduled total hip replacement under general anesthesia will be randomized into one of two arms: 1) the pre-closure group will receive the first dose administered upon onset of skin closure, 2) the preemptive group will receive the first dose administered within 60 minutes prior to incision. Both groups will receive subsequent doses every 6 hours x 24 hours.

The purpose of the study is to examine the effects of timing of administration of intravenous (IV) acetaminophen and how timing relates to analgesic efficacy as measured by postoperative pain scores. A maximum of 138 patients meeting inclusion criteria who present for a scheduled total hip replacement under general anesthesia will be randomized into one of two arms: 1) the closure group will receive the first dose of one gram of intravenous acetaminophen administered upon onset of skin closure, 2) the preemptive group will receive the first dose of one gram of intravenous acetaminophen administered within 60 minutes prior to incision. Both groups will receive subsequent doses of one gram of intravenous acetaminophen every 6 hours x 24 hours. The maximum dose in a 24 hour period will not exceed 4 grams according to FDA recommendations.

Inclusion Criteria:

• Patients 30-75 years of age weighing ≥55 kg scheduled for elective total hip arthroplasty performed under general anesthesia will be included. As determined by the anesthesia team assigned to the case, eligible patients will also be assigned ASA physical status of I, II, or III and deemed capable of reporting their perceived pain using numeric pain scales and capable of operating a patient controlled anesthesia (PCA) device.

Exclusion Criteria:

• Exclusion criteria include known allergy, hypersensitivity, or contraindication to acetaminophen, history of alcohol or drug abuse, prisoners, emergency THAs, patients with chronic malnutrition or a body mass index (BMI) < 18 kg/m2 and non-English speaking patients. Additional exclusion criteria include impaired liver function defined as AST and ALT each ≥ twice normal limits and renal dysfunction (creatinine >2.0 mg/dl).