Pharmacokinetics of methadone.

Methadone is a synthetic opioid that is effective for the relief of moderate-to-severe pain and for the treatment of opioid dependence. The pharmacokinetics of methadone differ from those of morphine in that methadone has a higher bioavailability, a much longer half-life, and is hepatically metabolized by cytochrome P450 enzymes. The pharmacokinetics of methadone are variable and an understanding of the factors that impact the onset, magnitude, and duration of analgesia is required to optimize therapy. Drug interactions are common and patients receiving methadone should be monitored closely for toxicity or therapeutic failure. Special populations in whom a change from the usual dosage regimen may be necessary include pediatric patients, patients with renal failure, the elderly, and pregnant women. To achieve an optimal dosage regimen, the clinician must have an understanding of the pharmacokinetics and pharmacodynamics of methadone in addition to the relationship between these variables and their patients' demographic and pathophysiologic characteristics. AMEDLINE search was performed to identify literature published between 1966 and May 2005 relevant to the pharmacokinetics of methadone. These publications were reviewed and the literature summarized regarding unique and clinically important elements of methadone disposition including its absorption profile, distribution, and metabolism/excretion. General dosing guidelines, dosage conversions from other opioids and pharmacokinetic issues in special populations are discussed.