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Half-life The half-life of a drug is the name given to the time it takes for blood levels of a drug to drop to 50% of the peak concentration. The half-life of diamorphine (heroin) is around 3 minutes. The half-life of methadone depends upon whether it is a first dose or a dose given as part of an ongoing programme. Single, first dose The apparent half-life of a single oral dose of methadone is shorter than that in extended use. This is because much of the initial dose becomes distributed into the tissue reservoirs and is therefore not available in the blood stream.
Following ingestion of oral methadone blood levels rise for about 4 hours and then begin to fall. The apparent half-life of a single first dose is 12-18 hours with a mean average of 15 hours.68 Single dose of oral methadone First few days of usage Over the first 3 days of methadone consumption the 'tissue reservoirs' of methadone in the lungs, kidneys and liver gradually fills. After the first day subsequent doses start from a higher baseline and therefore reach a higher peak. The half-life of the drug reflects only clearance of the drug from the system and is therefore extended to between 13 and 47 hours with a mean average of 25 hours.68
This graph illustrates the 3 days it takes for the 'tissue reservoirs' to fill. First 3 days of dispensing, for oral methadone, with once-daily dosing (doses at 0, 24, 48 hours) Regular dosing Once in a steady state variations in blood concentration levels are relatively small. Clients may prefer to take their daily dose at a particular time each day but this makes little difference to their blood levels of methadone.
Methadone's long half-life means that once-daily dosing should theoretically be adequate for clients who have been on a constant oral methadone dose for more than 3 days. Once-daily dosing of oral methadone at steady state Missed dose If one day's dose of methadone is omitted from a regular regime the blood concentration will continue to fall gradually over the 24-48 hour interval.
After 25 hours a person on a regular once-daily dosing regime will have methadone blood levels equal to around half the peak level i.e. 4 hours after dosing. The blood concentration would typically fall to 25% of the peak level after 48 hours. 3 day recovery to steady state from missed dose at day 10http://www.exchangesupplies.org/drug.../section4.html