Objective:To determine the effectiveness of an oral versus sublingual loading dose of nifedipine for tocolysis.Methods: An open, randomized clinical trial conducted between March 1, 2013, and April 31, 2014. Participants were pregnant women with a diagnosis of premature labor, single live fetus, topical pregnancy, gestational age 24-36 weeks, normal fetal vitality, cervical dilatation less than or equal to 4 cm, cervical effacement less than or equal to 80%, and intact amniotic membranes. They were randomized into two groups, oral and sublingual nifedipine, 20 mg loading dose, repeated every 30 minutes (maximum dose of 60 mg). The primary endpoint was the time until tocolysis and the secondary endpoints were the effectiveness of tocolysis within 90 minutes, 12 hours, and 48 hours; premature delivery within 48 hours; and maternal hemodynamic parameters and side effects. Results: There were 80 patients randomized to oral (n=40) and sublingual (n=40) nifedipine. The time required for tocolysis was significantly less with sublingual nifedipine (160 minutes vs 340 minutes; P=0.0003). Sublingual nifedipine was also more successful than oral nifedipine at inhibiting premature labor within 90 minutes (n=8 [20.0%] vs n=1 [2.5%], P=0.014). There was no statistically significant difference between the groups for the other secondary endpoints. Conclusion: Compared with oral administration, a sublingual loading dose of nifedipine resulted in faster tocolysis in patients with premature labor. Brazilian Clinical Trials Registry (ReBEC): U1111-11566186.