 Currently, the most widely used drugs to treat pain after surgery are opiates, such as morphine. Morphine works mainly by activating one of several types of opiate receptors that control some of our pain sensation - the so-called mu opiate receptors. These receptors are located in many areas of the brain and also outside of the brain. By activating these receptors, morphine provides significant pain relief, but also causes side effects that limit its use. Some of these side effects include: respiratory depression or arrest slowed or stopped breathing, sedation a state of calmness or extreme relaxation, euphoria an exaggerated feeling of physical and mental well-being, constipation, nausea, vomiting, and drug addiction. 