Background: Intrathecal Buprenorphine is an adjuvant to spinal anaesthesia offers the advantage of providing good analgesia, but is associated postoperative nausea and vomiting. Dexamethasone is a potent steroid and has good antiemetic effect. The aim of this randomized controlled trial was to compare the antiemetic efficacy of dexamethasone 4 and 8mg. Methods: 180 patients of either sex undergoing elective lower extremities and infra umbilical surgeries were included in the study. The patients in group D0, D4 and D8 received 5ml normal saline, 4 mg and 8mg dexamethasone respectively before spinal anaesthesia. Spinal anaesthesia was established with a single bolus of 0.5% hyperbaric bupivacaine 15 mg and buprenorphine 60mcg. The primary end point of this study was the total nausea and vomiting rate for 8 hours post spinal anaesthesia. The secondary end points were the incidence of nausea, vomiting and the occurrence of adverse events like pruritis. Statistical Analysis: The incidence of nausea, vomiting and pruritus was analyzed using a series of 3×2 2 tests. Results: Demographic profile was similar between the groups. The incidence of nausea was 16% in group D0 and was 5% in the groupD4 and D8. The incidence of vomiting was 5%, 3.3%and 1.7% in the D0, D4 and D8 groups respectively. The incidence of nausea and vomiting was not very different in the D4 and D8 groups. Conclusions: Both doses of prophylactic dexamethasone 4/8 mg significantly reduces the nausea and vomiting associated with intrathecal Buprenorphine. A dose of 4mg of dexamethasone is as efficacious as 8mg.