BackgroundManagement of analgesia and hemodynamic status is essential for orthopedic surgery patients. This study aimed to determine the effect of intrathecal (bupivacaine + magnesium sulfate) versus (pethidine + magnesium sulfate) and sole pethidine on the hemodynamic status and onset and duration of sensory/motor block in patients with lower limb fractures.MethodsThis double-blind randomized clinical trial was done between October 2020 and August 2021 in the Gorgan 5thAzar, Hospital, northeastern Iran and included 120 patients undergoing orthopedic surgery with spinal anesthesia and divided them in four groups of A: bupivacaine (10 mg), B: (pethidine 1 mg / kg), C: (bupivacaine 10 mg + magnesium sulfate 100 mg), and D: (pethidine 1 ‌mg / kg + magnesium sulfate 100 mg). The recorded parameters were hemodynamic parameters, time of onset, and duration of sensory/motor block.ResultsThere were no clinically significant differences in hemodynamic parameters between the groups. The onset of sensory block (P = 0.235), the onset of motor block (P = 0.097), and the duration of motor block (P = 0.135) were not statistically different between the groups; however, there was a significant difference in the duration of sensory block between the groups (P = 0.035). Magnesium sulfate increased the duration of motor block in the pethidine group C (80.93 ± 30.28) and decreased it in the bupivacaine group D (75.23 ± 38.56).ConclusionsThe intrathecal drugs used in the study did not cause any specific side effects. Therefore, these drugs can be used interchangeably. Magnesium sulfate, in combination with bupivacaine reduced the duration of sensory/motor block. While in the pethidine group, it acted as suitable analgesia by increasing the duration of sensory block. Therefore, magnesium can be considered a suitable additive for intrathecal pethidine.Trial registration: Iran clinical trial register: (IRCT20170413033408N3) Registration date: 2020-06-11