How is meth used as a medicine?

Which meth?: I actually HATE that word, because some people use it for methadone, and others for methamphetamine. Both are medications. Methadone is a pain medication, used in severe chronic pain as well as the treatment of opioid addiction. Methamphetamine (Desoxyn) is used to treat the same things Adderall or RItalin (methylphenidate) are used for - ADHD, narcolepsy, sleep shift disorders and conditions needing stimulants.