IV Acetaminophen as Adjuvant Analgesic to Hydromorphone - Emergency Department Patients

Inclusion Criteria: * Pain with onset within 7 days of the ED visit * ED attending physician's judgment that the patient's pain warrants IV opioids. * ED attending physician's judgment that the patient has capacity to provide informed consent. * ED attending physician's judgement that patient is not a chronic user of opioids or acetaminophen * Patients must be able to understand English or Spanish. Exclusion Criteria: * Use of opioids or tramadol within past 24 hours. * Use of acetaminophen or non-steroidal anti-inflammatory medication within the previous 8 hours. * Prior adverse reaction to opioids or acetaminophen. * Chronic pain syndrome: frequently recurrent or daily pain for at least 3 months; examples of chronic pain syndromes include sickle cell anemia, osteoarthritis, fibromyalgia, and peripheral neuropathies. * Medical condition that might affect metabolism or opioid analgesics or acetaminophen such as hepatitis, renal insufficiency or failure, hypo- or hyper-thyroidism, Addison's or Cushing's disease * Pregnant or breastfeeding * Alcohol intoxication: the presence of alcohol intoxication as judged by the treating physician or nurse * Not at risk of suicide assessed by triage nurse * Systolic blood pressure \<100 mmHg * Heart Rate \< 60/min * Oxygen saturation \< 95% on room air: * Use of monoamine oxidase (MAO) inhibitors in past 30 days * Use of transdermal pain patches * Taking any medication that might interact with one of the study medications, such as a selective serotonin reuptake inhibitor (SSRI) or tricyclic anti-depressants, antipsychotics, anti-malarial medications (quinidine or halofantrine), amiodarone or dronedarone, diphenhydramine, celecoxib, ranitidine, cimetidine, ritonavir, terbinafine or St. John's Wort. * Patients who have been previously enrolled in this same study