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Childbirth consists of a three-stage period beginning with uterine, contractions, and ending in the expulsion of the placenta. The aim of this study was the effect of intramuscular hyoscine and hyoscine-promethazine on the duration of labor in nulliparous pregnant women. This study was a triple-blind clinical trial on 213 nulliparous pregnant women who had referred to Amiralmomenin hospital in Zanjan, Iran. Selection of samples was random and done with using 3, 6(1:1:1) of blocking ratio. This study had three groups. The first group received 20 mg hyoscine (group A), the second group 3 cc sterile water (group B), and the third group 20 mg hyoscine and 25 mg promethazine (group C). Data collection tools included a demographic questionnaire, checklist of data, and partograph chart. The mean of the first stage of labor in hyoscine group was 88.95 ± 51.07 min, hyoscine-promethazine 100.05 ± 65.88 min, and control group 170.37 ± 106.87 min. There was significant correlation between A and C and B and C groups (P < 0.001). The mean of the second stage in group A was 46.37 ± 44.60, group B 32.25 ± 17.74, and control group 55.05 ± 44.44 min and a significant relationship was between B and C groups (P = 0.03). The results showed that hyoscine and hyoscine-promethazine can be used to reduce the duration of labor.