Drug Interactions between metformin and Methadone Diskets

GENERALLY AVOID: Alcohol may potentiate the central nervous system (CNS) depressant effects of methadone. Concomitant use may result in additive CNS depression and impairment of judgment, thinking, and psychomotor skills. In more severe cases, hypotension, respiratory depression, profound sedation, coma, or even death may occur.

GENERALLY AVOID: Coadministration with grapefruit juice may increase the plasma concentrations of methadone. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruits. In 8 study subjects stabilized on methadone maintenance treatment, ingestion of regular strength grapefruit juice (200 mL one-half hour before and 200 mL simultaneously with the daily methadone dose) for five days resulted in an approximately 17% mean increase in methadone peak plasma concentration (Cmax) and systemic exposure (AUC) and a 14% mean decrease in apparent clearance for both the R(+) and S(-) enantiomers. Grapefruit juice did not affect the time to peak level (Tmax), terminal half-life, or apparent volume of distribution of methadone. No signs or symptoms of methadone toxicity or changes in intensity of withdrawal symptoms were reported in the study. Pharmacokinetic interactions involving grapefruit juice are also subject to a high degree of interpatient variability, thus the extent to which a given patient may be affected is difficult to predict. In addition, high dosages (particularly above 200 mg/day) and high serum levels of methadone have been associated with QT interval prolongation and torsade de pointes arrhythmia.

MANAGEMENT: Patients should not consume alcoholic beverages or use drug products that contain alcohol during treatment with methadone. Any history of alcohol or illicit drug use should be considered when prescribing methadone, and therapy initiated at a lower dosage if necessary. Patients should be closely monitored for signs and symptoms of sedation, respiratory depression, and hypotension. In addition, patients treated with oral methadone should preferably avoid or limit the consumption of grapefruit juice, particularly during the induction of maintenance treatment. Given the interindividual variability in the pharmacokinetics of methadone, a significant interaction with grapefruit juice in certain patients cannot be ruled out. Patients should be advised to seek immediate medical attention if they experience symptoms that could indicate the occurrence of torsade de pointes such as dizziness, lightheadedness, fainting, palpitation, irregular heart rhythm, shortness of breath, or syncope.

GENERALLY AVOID: Alcohol can potentiate the effect of metformin on lactate metabolism and increase the risk of lactic acidosis. In addition, alcohol may cause hypoglycemia or hyperglycemia in patients with diabetes. Although hypoglycemia rarely occurs during treatment with metformin alone, the risk may increase with acute consumption of alcohol. Even modest amounts can lower blood sugar significantly, especially when the alcohol is ingested on an empty stomach or following exercise. The mechanism involves inhibition of both gluconeogenesis as well as the counter-regulatory response to hypoglycemia. Episodes of hypoglycemia may last for 8 to 12 hours after ethanol ingestion. By contrast, chronic alcohol abuse can cause impaired glucose tolerance and hyperglycemia. Moderate alcohol consumption generally does not affect blood glucose levels in patients with well controlled diabetes.

Food may have varying effects on the absorption of metformin from immediate-release versus extended-release formulations. When a single 850 mg dose of immediate-release metformin was administered with food, mean peak plasma concentration (Cmax) and systemic exposure (AUC) decreased by 40% and 25%, respectively, and time to peak plasma concentration (Tmax) increased by 35 minutes compared to administration under fasting conditions. By contrast, administration of extended-release metformin with food increased AUC by 50% without affecting Cmax or Tmax, and both high- and low-fat meals had the same effect. These data may not be applicable to formulations that contain metformin with other oral antidiabetic agents.

MANAGEMENT: Metformin should be taken with meals, and excessive alcohol intake should be avoided during treatment. Diabetes patients in general should avoid consuming alcohol if their blood glucose is not well controlled, or if they have hypertriglyceridemia, neuropathy, or pancreatitis. Alcohol should not be consumed on an empty stomach or following exercise, as it may increase the risk of hypoglycemia. Patients should contact their physician immediately if they experience potential signs and symptoms of lactic acidosis such as malaise, myalgia, respiratory distress, increasing somnolence, and nonspecific abdominal distress (especially after stabilization of metformin therapy, when gastrointestinal symptoms are uncommon). With more marked acidosis, there may also be associated hypothermia, hypotension, and resistant bradyarrhythmias. Metformin should be withdrawn promptly if lactic acidosis is suspected. Serum electrolytes, ketones, blood glucose, blood pH, lactate levels, and blood metformin levels may be useful in establishing a diagnosis. Lactic acidosis should be suspected in any diabetic patient with metabolic acidosis lacking evidence of ketoacidosis (ketonuria and ketonemia).