The goal of this clinical trial is to learn if ketamine works to reduce craving for opioids in adults entering methadone treatment for opioid use disorder. The main questions it aims to answer are: * Does ketamine reduce craving for opioids in patients with opioid use disorder? * Does ketamine reduce symptoms of opioid withdrawal such as depression, pain, and poor sleep quality? * Do patients who take the low dose ketamine stay in methadone treatment longer, and/or have better treatment outcomes than those given the very low dose? Researchers will compare two low doses of ketamine to see if ketamine works to reduce craving for opioids in adults entering methadone treatment for opioid use disorder. Participants will: * Be given a low dose or a very low dose of ketamine 4 times over a period of 2 weeks * Visit the clinic weekly and monthly for checkups and tests for 90 days post-intake