Background: To date, magnesium sulphate (MgSO4) is the treatment of choice for prevention of seizure in eclampsia and preeclampsia. However, there are some limitations in the administration of MgSO4 due to its tocolytic effects. The aim of this study was to compare the anticonvulsant and tocolytic effects of MgSO4 and another drug, phenytoin, in patients with eclampsia and preeclampsia.Methods: This clinical trial was conducted on pregnant women hospitalised with eclampsia or preeclampsia, during 2014–2016. The subjects were randomly assigned to two treatment groups using blocking method based on disease (eclampsia or mild and severe preeclampsia). One group received MgSO4 (group M) and another group received phenytoin (group P) as treatment. Each group consisted of 110 and 65 women with mild and severe preeclampsia, respectively (subgroup A), and 25 women with eclampsia (subgroup B). Duration of labor, the number of cesarean sections, convulsions and Apgar scores of infants were compared between the two groups and were considered as treatment outcomes.Results: Convulsion rate was significantly lower with MgSO4 than phenytoin (P = 0.001). No seizure occurred in patients with mild preeclampsia in group P. Duration of stage one of labor (P < 0.001) and the number of cesarean sections (P = 0.040) were significantly higher in group M. However, one-minute Apgar scores for newborns were higher in women treated with MgSO4 compared to that of phenytoin (P = 0.001). Five-minute Apgar was not significantly different.Conclusion: Although MgSO4 is more effective than phenytoin for prevention of convulsion in eclampsia and severe preeclampsia, phenytoin may be considered for treatment of special conditions such as mild preeclampsia. Due to the tocolytic effects of MgSO4 on increasing the duration of labor, the increased risk of cesarean section and the potential for toxicity, physicians should critically consider the best drug according to the condition of the patient.