The goal of this randomized clinical trial is to compare the analgesic efficacy and side effect profile of IV acetaminophen as an analgesic adjunctive medication to IV hydromorphone for the treatment of acute pain experienced by patients in the Emergency Department (ED).

Multimodal treatment of pain is a commonly used strategy to control pain by combining analgesics with different and complementary mechanisms of action. Oral acetaminophen combined with opioids is a mainstay of treatment of mild to moderate pain, while the use of IV opioids is the standard for treatment of severe pain in the Emergency Department. An IV formulation of acetaminophen is widely used in Europe and has recently been approved in the US. It has been studied for treatment of acute post-operative pain and renal colic.

This randomized trial was designed to assess the effectiveness of the combination of IV acetaminophen and IV hydromorphone as a strategy to provide more effective treatment of acute severe pain in the Emergency Department with the possibility of reducing opioid consumption. All patients received 1 mg of IV hydromorphone. They were then randomized to receive either 1 g of IV acetaminophen or 100 mL of normal saline placebo. Measures of pain, vital signs, and presence of side effects were obtained immediately before IV hydromorphone was administered. Following receipt of the acetaminophen or placebo these same variables were measured at 15 minute intervals for the next 120 minutes. Additional analgesics were administered if patients requested them. Subsequent care of the patients was at the discretion of the treating attending physician.

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

During the entire study - within 2 hours after administration of study medication

The pharmacist working in an area inaccessible to ED staff will ensure blinding by creating identical vials containing either 1000 mg acetaminophen in 100 ml normal saline or 100 ml normal saline placebo and numbering them sequentially for use in this study.