Magnesium sulfate has been shown to be a successful tool in a large number of clinical areas. Its benefits include neuroprotection, control of eclampsia / pre-eclampsia, control of intraoperative blood pressure, decreased neuroendocrine response during laryngoscopy and tracheal intubation and reduced levels of postoperative pain and consumption of analgesic. Obese patients have become more and more frequent in the operating rooms, due to the increasing prevalence of this population worldwide. However, although they have received magnesium sulfate as part of the analgesic strategy in many centers, there has been no study demonstrating the appropriate dose of this medication in obese patients. This study aims to compare two doses of magnesium sulfate in obese patients: based on their real weight or based on ideal weight.