Drug Interactions between Cartia XT and Riomet

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).

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MONITOR: Like many CNS-active agents, alcohol can exhibit hypotensive effects. Coadministration with antihypertensive agents including diltiazem may result in additive effects on blood pressure and orthostasis.

MONITOR: Grapefruit juice may increase the plasma concentrations of orally administered diltiazem in some patients. The proposed mechanism is inhibition of CYP450 3A4-mediated first-pass metabolism in the gut wall by certain compounds present in grapefruit. In a study of ten healthy male volunteers, administration of a single 120 mg oral dose of immediate-release diltiazem in combination with 250 mL of grapefruit juice increased the diltiazem peak plasma concentration (Cmax) and systemic exposure (AUC) by an average of 22% and 20%, respectively, compared to administration with water. The time to reach Cmax (Tmax) and the terminal half-life were not affected, and no statistically significant differences in blood pressure and heart rate were observed during administration with grapefruit juice relative to water. In a different study, repeated administration of 200 mL of grapefruit juice at 0, 2, 4, 8 and 12 hours had no significant effect on the Cmax or AUC of a single 120 mg oral dose of diltiazem, but increased its half-life from 4.1 to 5.1 hours. The ratios for the N-demethyl and deacetyl metabolites to diltiazem were also not affected by grapefruit juice. However, because pharmacokinetic interactions involving grapefruit juice are often subject to a high degree of interpatient variability, the extent to which a given patient may be affected is difficult to predict.

MANAGEMENT: Patients should be advised that alcohol may potentiate the hypotensive effects of diltiazem, especially during the initiation of therapy and following a dosage increase. Caution should be exercised when rising from a sitting or recumbent position, and patients should notify their physician if they experience dizziness, lightheadedness, syncope, orthostasis, or tachycardia. Patients who regularly consume grapefruit or grapefruit juice should be monitored for increased adverse effects of diltiazem such as such as headache, irregular heartbeat, edema, unexplained weight gain, and chest pain. Grapefruit and grapefruit juice should be avoided if an interaction is suspected.

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MONITOR: Calcium-containing products may decrease the effectiveness of calcium channel blockers by saturating calcium channels with calcium. Calcium chloride has been used to manage acute severe verapamil toxicity.

MANAGEMENT: Management consists of monitoring the effectiveness of calcium channel blocker therapy during coadministration with calcium products.

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