Pain is a common element of surgery. Opiates (morphine, oxycodone, hydrocodone, methadone, fentanyl) are very helpful in decreasing pain after surgery. Unfortunately, with repeated use opiates lose their effectiveness, such that patients need to utilize more opiates to achieve adequate pain relief - a phenomenon called tolerance. Sometimes tolerance to a pain reliever's effects can develop in just a few hours. It is thought that activation of the N-methyl d-aspartate (NMDA) receptor, a "switch" found on the surface of nerves, is partially responsible for opiate tolerance. Memantine is a medication that limits the activity of NMDA receptors in the brain and spinal cord. It has been used for years to help patients with Alzheimer's Disease. In this study, we will study the effects of memantine when combined with opiate medications to see whether it can increase the effectiveness of opiates for pain after surgery and reduce the side effects caused by opiates (e.g., sedation, nausea, itching).