Physeptone is a brand name for the generic drug methadone, Other brand or trade-names for the same drug are Methadose, Dolophine, Heptadon.
Methadone is a morphine-like drug used to treat severe pain and to prevent withdrawal symptoms in people who are addicted to heroin or other opiates. It may also be used to treat severe coughing in lung cancer sufferers. Methadone belongs to the family of drugs called "opioid analgesics".
IMHO anyone taking methadone in the medium to long-term is not an ex-drug addict, they are simply addicted to a different drug, and for many people who use methadone long-term, it is a more difficult drug to get clean of than heroin.