Methadone is an opioid analgesic and belongs to the weak opioid class. Methadone can treat mild to moderate pain. Methadone is currently used abroad as an alternative drug to drug treatment, including maintenance drugs in the drug treatment process, and fentanyl and morphine are mu agonists, these drugs have side effects, such as mu receptors in addition to bouts of pain, nausea, vomiting, pupil narrowing, inhibition of gastrointestinal motility, etc., but methadone addiction is relatively low, in foreign countries mainly for drug treatment use. Methadone is currently used more in foreign countries and is used more in pain treatment and substitution therapy, while it also belongs to the general μ-receptor effect of opioids. Its side effects include general opioid side effects, such as respiratory depression, nausea, vomiting, and even pupil pinpoint changes after high dosage.