The investigator team proposes a randomized, double-blind, double-dummy comparative effectiveness trial conducted in two urban emergency departments (EDs) in the Bronx, New York. This study is designed to determine the relative efficacy of IV acetaminophen compared to PO acetaminophen in treating pelvic pain. This design focuses on the early onset of action and short-term efficacy, which may better capture potential differences between IV and PO acetaminophen in the acute ED setting.

An estimated 70% of Emergency Department (ED) visits involve pain as a complaint. Although ED practice has shifted away from routine opioid prescribing, uncertainty remains regarding optimal selection among commonly used non-opioid analgesics such as nonsteroidal anti-inflammatory drugs (NSAIDs) and acetaminophen. Medication selection varies by pain etiology, and among patients presenting with musculoskeletal pain, opioids (40.7%), acetaminophen (37.8%), and NSAIDs (22.6%) remain the most frequently administered medications in the ED.

Pain in women has been comparatively understudied. Pelvic pain is common among women of childbearing age, and chronic pelvic pain affects up to 24% of women overall. In nonpregnant women, NSAIDs are widely considered first-line therapy for both acute and chronic pelvic pain. In pregnant women and in those attempting to conceive, NSAIDs are typically avoided. Observational studies have associated NSAID use around the time of conception or prior to 20 weeks' gestation with an increased risk of miscarriage, while acetaminophen has not shown a similar association. NSAID exposure in early pregnancy has also been linked to congenital anomalies.

Guidelines recommend limiting opioid use during pregnancy and in women of childbearing age. Opioid exposure has been associated with congenital anomalies and with poorer maternal and neonatal outcomes. As a result, opioids are generally avoided as first-line therapy for pelvic pain in patients who are pregnant or may be pregnant.

Therefore, it is routine to ascertain pregnancy status prior to administering NSAIDs or opioids to women of childbearing age for an informed decision making discussion. Acetaminophen, in contrast, is generally considered safe in pregnancy and can be administered without delay while awaiting pregnancy testing. Acetaminophen is associated with relatively mild side effects, which may vary by route of administration.

Pharmacokinetic studies demonstrate that intravenous acetaminophen achieves higher peak plasma concentrations and faster central nervous system penetration than oral administration. Outside the ED, IV acetaminophen has been associated with faster onset of meaningful pain relief and reduced opioid use in some surgical populations. Whether these pharmacologic advantages translate into clinically meaningful improvements in acute pelvic pain management in the Emergency Department for patients of childbearing potential with pelvic pain is unclear.

The investigator team hypothesizes that among women aged 16-50 presenting to the emergency department with pelvic pain, patients receiving intravenous acetaminophen will achieve a greater improvement in the numeric rating scale (NRS) pain score at 30 minutes compared with oral acetaminophen.

Inclusion Criteria:

• Female sex at birth
• Presentation to the Emergency Department (ED) with pelvic pain
• Baseline numeric pain score (NRS) ≥4
• Ability to provide informed consent in English or Spanish

Exclusion Criteria:

• Receipt of any analgesic medication within 2 hours or acetaminophen within 6 hours
• Known allergy or intolerance to acetaminophen

Allocation concealment will be maintained using sequentially numbered medication kits prepared by the research pharmacy. Participants, treating clinicians, research staff, outcome assessors, and investigators will remain blinded to treatment allocation.