Module 2: Basic Pharmacology of Controlled Drugs and Substances
Methadone – Pharmacokinetics
When taken orally, methadone has a 90% bioavailability and a long half-life (36-40 hours in some patients), which facilitates once daily dosing for patients using methadone for addiction management. It can be given more frequently in smaller doses for chronic pain. Methadone is lipid soluble and binds to plasma proteins. Methadone is metabolized in the liver through cytochrome P450 isoenzymes and excreted in bile and urine. Because its metabolites are inactive, it is particularly useful for patients with renal impairment or failure. Respiratory depression and prolonged QT intervals can occur if the dose is too high. The aforementioned half-life can also create risks to patients who are opioid naïve or who have missed regular doses of their methadone due to the risk of overdosing.