Background: Postsurgical pain is the main cause of anxiety in patients; therefore, analgesics (adjuvants) such as preemptive doses of ketamine with minimal adverse effects would be beneficial. However, studies are needed regarding their efficacy. Aim: To determine the preemptive effect of intravenous ketamine on pain intensity and need to opioids in cesarean section which performed under spinal anesthesia. Subjects and Methods: The study was a randomized, doubleâ€‘blinded, clinical trial involving 60 term parturients for cesarean, using random block method, they were divided into two groups of each. The case group received ketamine with dose of 0.2 mg/kg and the control one normal saline with the same volume. Pain intensity was compared in 0,30,60,90,120,150, and 180 min and 6,12,18, and 24 h after surgeries with visual analog scale (VAS) index. The average opioid usage was compared during 24 h after those too. Final analyses were done with Mannâ€‘Whitney, Chiâ€‘square, and Spss.v. 16 (P < 0.05 was meaningful level). Results: There was not significant statistical difference on average VAS during interrupted times (F = 0.15, P = 0.70). Average dosage of diclofenac suppository and mean time for taking the first dosage of opioids have not statistical difference too (respectively; P = 0.76, P = 0.87). Average dose of pethidine was lesser than placebo statistically. It means, the case group did not take pethidine but this amount was 6 (20%) in the control one (P = 0.02). Conclusion: Taking the preemptive dosage of ketamine (0.2 mg/kg) before cesarean could act as a probably model for decreasing opioid consumption.