A methadone clinic is where serious addicts go to get off narcotics/opioids, it's pretty rare for people to abuse methadone since there's more powerful stuff more readily available (heroin, fentanyl, et al). The withdrawals are killer so methadone helps them work off the stuff in a safer manner. Sometimes it's given as "pain management" for people who have spent a long time in hospital on serious drugs after a bad injury/massively invasive surgery, but the wink and nudge tells them to use it to wean themselves off the addiction.