Because methadone requires time to build up in the system, if a person misses dosages they may need to return to a lower level.
As a rule of thumb I've learned that if a person misses three days of methadone and is on 100 mg the dosage is halved, ie 50 and 25% added, roughly 70 mg. If a person misses 4 days then they probably will have to start back at the 20 to 40 mg dosage that is used as a starting dosage. A person who has an unknown tolerance for methadone may be started at 15 to 25 mg a day to be safe. Overdose deaths have occurred with dosages as low at 50 mg in non tolerant individuals. When one misses dosages for more they lose their tolerance for methadone.
Methadone routinely is increased by 5-10 mg a week, no more frequently than every 3-5 days. Therefore a person missing dosages may have this set back where once again they have to build up to the most effective dosage for them.
The maximum dosages for starting a new person, which a person off methadone for 4 days essentially, becomes is 40 mg.