Drug Interaction Report

MONITOR: The concomitant administration of magnesium salts with other magnesium-containing preparations such as antacids or laxatives may increase the risk of magnesium toxicity, particularly in the presence of renal insufficiency. Magnesium toxicity may result in respiratory depression, neuromuscular depression, and heart block.

MANAGEMENT: Clinical and laboratory monitoring for signs of hypermagnesemia and magnesium toxicity is advisable, especially in high-risk patients (e.g., elderly or renally impaired patients).

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Antacids and some oral aluminum, calcium, or magnesium containing preparations may increase the absorption of levodopa. The mechanisms may be related to increased gastric pH and/or decreased gastric emptying time, resulting in less gastric degradation of levodopa and more levodopa absorption from the small intestine. This effect may be beneficial in some patients, but also difficult to predict and monitor. For most patients, this interaction is not clinically significant and no specific intervention would be required. Dosing intervals could be spaced by at least 2 hours if it is preferred to avoid this interaction.

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MONITOR: Vitamin K may antagonize the hypoprothrombinemic effect of oral anticoagulants. Vitamin K is a cofactor in the synthesis of blood clotting factors that are inhibited by oral anticoagulants, thus intake of vitamin K through supplements or diet can reverse the action of oral anticoagulants. Resistance to oral anticoagulants has been associated with consumption of foods or enteral feedings high in vitamin K content. Likewise, a reduction of vitamin K intake following stabilization of anticoagulant therapy may result in elevation of the INR and bleeding complications. Foods rich in vitamin K include beef liver, broccoli, Brussels sprouts, cabbage, collard greens, endive, kale, lettuce, mustard greens, parsley, soy beans, spinach, Swiss chard, turnip greens, watercress, and other green leafy vegetables. Moderate to high levels of vitamin K are also found in other foods such as asparagus, avocados, dill pickles, green peas, green tea, canola oil, margarine, mayonnaise, olive oil, and soybean oil. Snack foods containing the fat substitute, olestra, are fortified with 80 mcg of vitamin K per each one ounce serving so as to offset any depletion of vitamin K that may occur due to olestra interference with its absorption. Whether these foods can alter the effect of oral anticoagulants has not been extensively studied. One small study found that moderate consumption (1.5 servings/day) does not significantly affect the INR after one week in patients receiving long-term anticoagulation.

Consumption of large amounts of mango fruit has been associated with enhanced effects of warfarin. The exact mechanism of interaction is unknown but may be related to the vitamin A content, which may inhibit metabolism of warfarin. In one report, thirteen patients with an average INR increase of 38% reportedly had consumed one to six mangos daily 2 to 30 days prior to their appointment. The average INR decreased by 17.7% after discontinuation of mango ingestion for 2 weeks. Rechallenge in two patients appeared to confirm the interaction.

Limited data also suggest a potential interaction between warfarin and cranberry juice resulting in changes in the INR and/or bleeding complications. The mechanism is unknown but may involve alterations in warfarin metabolism induced by flavonoids contained in cranberry juice. At least a dozen reports of suspected interaction have been filed with the Committee on Safety of Medicines in the U.K. since 1999, including one fatality. In the fatal case, the patient's INR increased dramatically (greater than 50) six weeks after he started drinking cranberry juice, and he died from gastrointestinal and pericardial hemorrhage. However, the patient was also taking cephalexin for a chest infection and had not eaten for two weeks prior to hospitalization, which may have been contributing factors. Other cases involved less dramatic increases or instabilities in INR following cranberry juice consumption, and a decrease was reported in one, although details are generally lacking. In a rare published report, a 71-year-old patient developed hemoptysis, hematochezia, and shortness of breath two weeks after he started drinking 24 ounces of cranberry juice a day. Laboratory test results on admission revealed a decrease in hemoglobin, an INR greater than 18, and prothrombin time exceeding 120 seconds. The patient recovered after warfarin doses were withheld for several days and he was given packed red blood cells, fresh-frozen plasma, and subcutaneous vitamin K. It is not known if variations in the constituents of different brands of cranberry juice may affect the potential for drug interactions.

There have been several case reports in the medical literature of patients consuming grapefruit, grapefruit juice, or grapefruit seed extract who experienced increases in INR. R(+) warfarin, the less active of the two enantiomers of warfarin, is partially metabolized by CYP450 3A4. Depending on brand, concentration, dose and preparation, grapefruit juice may be considered a moderate to strong inhibitor of CYP450 3A4, thus coadministration with warfarin may decrease the clearance of R(+) warfarin. However, the clinical significance of this effect has not been established. A pharmacokinetic study found no effect on the PT or INR values of nine warfarin patients given 8 oz of grapefruit juice three times a day for one week.

A patient who was stabilized on warfarin developed a large hematoma in her calf in association with an elevated INR of 14 following consumption of approximately 3 liters of pomegranate juice in the week prior to admission. In vitro data suggest that pomegranate juice can inhibit CYP450 2C9, the isoenzyme responsible for the metabolic clearance of the biologically more active S(-) enantiomer of warfarin. In rats, pomegranate juice has also been shown to inhibit intestinal CYP450 3A4, the isoenzyme that contributes to the metabolism of R(+) warfarin.

Black currant juice and black currant seed oil may theoretically increase the risk of bleeding or bruising if used in combination with anticoagulants. The proposed mechanism is the antiplatelet effects of the gamma-linolenic acid constituent in black currants.

Soy protein in the form of soy milk was thought to be responsible for a case of possible warfarin antagonism in an elderly male stabilized on warfarin. The exact mechanism of interaction is unknown, as soy milk contains only trace amounts of vitamin K. Subtherapeutic INR values were observed approximately 4 weeks after the patient began consuming soy milk daily for the treatment of hypertriglyceridemia. No other changes in diet or medications were noted during this time. The patient's INR returned to normal following discontinuation of the soy milk with no other intervention.

An interaction with chewing tobacco was suspected in a case of warfarin therapy failure in a young male who was treated with up to 25 to 30 mg/day for 4.5 years. The inability to achieve adequate INR values led to eventual discontinuation of the chewing tobacco, which resulted in an INR increase from 1.1 to 2.3 in six days. The authors attributed the interaction to the relatively high vitamin K content in smokeless tobacco.

MANAGEMENT: Intake of vitamin K through supplements or diet should not vary significantly during oral anticoagulant therapy. The diet in general should remain consistent, as other foods containing little or no vitamin K such as mangos and soy milk have been reported to interact with warfarin. Some experts recommend that continuous enteral nutrition should be interrupted for one hour before and one hour after administration of the anticoagulant dose and that enteral formulas containing soy protein should be avoided. Patients should also consider avoiding or limiting the consumption of cranberry juice or other cranberry formulations (e.g., encapsulated dried cranberry powder), pomegranate juice, black currant juice, and black currant seed oil.

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GENERALLY AVOID: Alcohol may potentiate some of the pharmacologic effects of levodopa. Use in combination may result in additive central nervous system depression and/or impairment of judgment, thinking, and psychomotor skills.

MONITOR: Limited clinical data suggest that high protein content in the diet may reduce or cause fluctuations in the clinical response to oral and enteral formulations of levodopa in patients with Parkinson's disease. Proposed mechanisms include delayed gastric emptying, decreased levodopa absorption when taken with a protein rich diet, and competition with certain amino acids for transport across the gut wall and/or the blood brain barrier. Data have been conflicting. Clinical studies have variously reported no effect, reduced levodopa absorption with low-protein meals, reduced effects of oral and enteral formulations of levodopa with high daily protein intake, and no differences compared to fasting with high-protein meals. Neuroleptic malignant-like symptoms were reported in a patient with Parkinson's disease who was receiving pramipexole, entacapone, and immediate-release levodopa/carbidopa, after the protein content of his enteral feedings via nasogastric tube was increased from 0.88 g/kg/day to 1.8 g/kg/day; symptoms improved after the protein was reduced to 1 g/kg/day and bromocriptine was administered. Another patient receiving immediate-release carbidopa/levodopa, pramipexole, and entacapone experienced severe rigidity after initiation of continuous enteral nutrition via oral gastric tube containing 1.4 g/kg/day of protein; his Parkinsonian symptoms improved after the protein content was reduced to 0.9 g/kg/day, the feeding was changed to bolus feedings, and the levodopa was administered between boluses.

MANAGEMENT: In general, alcohol consumption should be avoided or limited during treatment with CNS-depressant agents. Until more data are available, it is advisable to avoid large fluctuations in daily protein intake and to monitor patients for altered effects of oral and enteral levodopa formulations if the protein content of the diet is increased.

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MONITOR: Enhanced hypoprothrombinemic response to warfarin has been reported in patients with acute alcohol intoxication and/or liver disease. The proposed mechanisms are inhibition of warfarin metabolism and decreased synthesis of clotting factors. Binge drinking may exacerbate liver impairment and its metabolic ability in patients with liver dysfunction. The risk of bleeding may be increased. Conversely, reductions in INR/PT have also been reported in chronic alcoholics with liver disease. The proposed mechanism is that continual drinking of large amounts of alcohol induces the hepatic metabolism of anticoagulants. Effects are highly variable and significant INR/PT fluctuations are possible.

MANAGEMENT: Patients taking oral anticoagulants should be counseled to avoid large amounts of ethanol, but moderate consumption (one to two drinks per day) are not likely to affect the response to the anticoagulant in patients with normal liver function. Frequent INR/PT monitoring is recommended, especially if alcohol intake changes considerably. It may be advisable to avoid oral anticoagulant therapy in patients with uncontrollable drinking problems. Patients should be advised to promptly report any signs of bleeding to their doctor, including pain, swelling, headache, dizziness, weakness, prolonged bleeding from cuts, increased menstrual flow, nosebleeds, bleeding of gums from brushing, unusual bleeding or bruising, red or brown urine, or red or black stools.

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MONITOR: Multivitamin preparations containing vitamin K may antagonize the hypoprothrombinemic effect of oral anticoagulants in some patients. Vitamin K1 in its active, reduced form serves as a cofactor in the generation of functional clotting factors, during which it becomes oxidized. It is reactivated in a process that is inhibited by oral anticoagulants, thus intake of additional vitamin K through supplements or diet can reverse the action of oral anticoagulants. Although the amount of vitamin K in over-the-counter multivitamin preparations is generally well below the dose thought to affect anticoagulation, there have been isolated case reports of patients stabilized on warfarin whose INR decreased following initiation of a multivitamin supplement and returned to therapeutic levels upon cessation of the multivitamin. Increases in warfarin dosage were required in some cases when the multivitamin was continued. One patient whose warfarin dosage was increased developed a subcapsular hematoma in her right kidney two weeks after she discontinued the multivitamin without informing her physician. Her INR was 13.2 and she was treated with vitamin K and fresh frozen plasma. It is possible that patients with low vitamin K status may be particularly susceptible to the interaction. Investigators have shown that vitamin K deficiency can cause an oversensitivity to even small increases in vitamin K intake. In one study where warfarin-stabilized patients were given a multivitamin tablet containing 25 mcg of vitamin K1 daily for 4 weeks, subtherapeutic INRs occurred in 9 of 9 patients with low vitamin K1 levels (<1.5 mcg/L) and only 1 of 7 patients with normal vitamin K1 levels (>4.5 mcg/L). INR decreased by a median of 0.51 and warfarin dosage had to be increased by 5.3% in patients with low vitamin K1 levels, whereas INR and warfarin dosage did not change significantly in patients with normal vitamin K1 levels. The prevalence of vitamin K deficiency may be small, but significant in the anticoagulated population. In a survey of 179 consecutive ambulatory patients on stable warfarin therapy attending an anticoagulation clinic, 22 (12.3%) were found to have vitamin K1 deficiency (<0.1 ng/mL).

MANAGEMENT: The potential for multivitamin supplements containing even low levels of vitamin K to affect anticoagulation should be recognized. In particular, elderly and/or malnourished patients may require more frequent monitoring of INR following the initiation or discontinuation of a multivitamin supplement, and the anticoagulant dosage adjusted as necessary.

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ADJUST DOSING INTERVAL: The oral bioavailability and pharmacologic effects of levodopa and carbidopa may be decreased during concurrent administration with iron-containing products. The proposed mechanism is chelation of levodopa and carbidopa by the iron cation, forming an insoluble complex that is poorly absorbed from the gastrointestinal tract. In nine patients with Parkinson's disease, administration of levodopa-carbidopa 100 mg-25 mg with ferrous sulfate 325 mg decreased levodopa peak plasma concentration (Cmax) and systemic exposure (AUC) by 47% and 30%, respectively, and carbidopa Cmax and AUC by 77% and 82%, respectively, compared to administration with placebo. There was also evidence of reduced efficacy of levodopa in some patients. In another study consisting of eight healthy subjects, coadministration of levodopa 250 mg with ferrous sulfate 325 mg resulted in greater than 50% reductions in the Cmax and AUC of levodopa compared to administration of levodopa alone. The magnitude of the interaction was the greatest in patients whose plasma levels of levodopa were the highest following administration of levodopa alone.

MANAGEMENT: Until more information is available, patients receiving levodopa and/or carbidopa in combination with iron-containing products should be advised to separate the times of administration by as much as possible. Patients should be monitored for reduced efficacy of levodopa, and the dosage adjusted as necessary.

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The recommended maximum number of medicines in the 'stimulant and hyperosmotic laxatives' category to be taken concurrently is usually one. Your list includes two medicines belonging to the 'stimulant and hyperosmotic laxatives' category:

• magnesium citrate
• polyethylene glycol 3350

Note: In certain circumstances, the benefits of taking this combination of drugs may outweigh any risks. Always consult your healthcare provider before making changes to your medications or dosage.