When Dilantin is taken concurrently with methadone, then blood levels of methadone go down and hence higher doses of methadone are needed.
People on methadone must avoid smoking for several reasons: (1) like methadone, it has adverse effects on lung/respiratory function and hence the side effects get added up, (2) smoking (nicotine) is itself an addicting agent; hence while one addiction (opioid) is being tackled with methadone, another addiction is being introduced into the body and (3) more significantly, the withdrawal effects of smoking (nicotine) are similar to those of opioid agents which not only complicates the treatment but can confuse the clinician into increasing the dose of methadone.