Objective To compare the effectiveness of intravenous acetaminophen with that of morphine in reducing pain in the first stage of labor. Methods An open-label, randomized controlled trial of women ≥ 34 weeks gestation in the first stage of labor, assigned to either intravenous acetaminophen or morphine. The primary outcome was improved analgesia measured by difference of visual analog scale (VAS) score at 120 minutes from baseline. Secondary outcomes were request for rescue analgesia, maternal side effects, and fetal heart rate changes. Statistical analyses performed were chi-square, Student's t-test, and Kaplan-Meier survival analysis. Results Of 40 women randomized, 18 received acetaminophen (2 did not receive study drug), and 20 received morphine. Because of difficulties in recruitment, the sample size of 88 was not achieved. The primary outcome was similar between groups (p = 0.53). Within 120 minutes of initial treatment, more women receiving intravenous acetaminophen required rescue analgesia (acetaminophen: 52.9% vs. morphine: 17.6%, p < 0.01). Maternal and fetal side effects were similar between groups. Conclusion There was no difference in VAS scores between groups. However, as half of women receiving intravenous acetaminophen required rescue analgesia within 120 minutes of treatment, intravenous acetaminophen may be less effective for analgesia in early labor compared with intravenous morphine.
Trial registration: ClinicalTrials.gov NCT02267772.
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