Get help from a doctor now ›. Methadone: is a long acting m opioid receptor agonist. The half-life of (R, S)- methadone is 15-60 hours. It takes approximately 6 half lives for a drug to be eliminated from your system. 60*6/24=15 days. The Methadone will stay in your system for about 15 days.Hope it helps. week or more: methadone is a very long-acting drug and is very slowly metabolized. The parent drug can stick around for 5-10 days in urine after a single dose, and metabolites up to a week longer. If you are prescribed it for pain, you need a note from your doctor.