| Fluconazole, like other azoles, may interfere with the metabolism of some drugs if given concomitantly, mainly through inhibition of the cytochrome P450 isoenzymes CYP3A4 and CYP2C9. The vast majority of formal interaction studies and case reports are related to multiple dose fluconazole use, therefore, the magnitude of the effect of this inhibition on an individual patient after a single dose of fluconazole is hard to predict, particularly in light of the individual variability in the activity of the isoenzymes. Nonetheless, single dose pharmacokinetic studies have demonstrated that the inhibitory action of fluconazole is immediate and leads, dose-dependently, to increased plasma concentrations of the interacting agents. Given fluconazole's long plasma elimination half-life of approximately 30 hours and substantially longer tissue bioavailability (see section 5.2 Pharmacokinetic Properties), these interactions may be clinically relevant following coadministration with drugs that have both a narrow therapeutic window and also act on vital organ systems like the heart and brain or are involved with glucose metabolism. Anticoagulants: Fluconazole increased the prothrombin time after warfarin administration in healthy males during an interaction study. The change was small (12%) but bleeding events such as bruising, epistaxis, gastrointestinal bleeding, hematuria and melena have been reported in association with increases in prothrombin time in patients receiving fluconazole and warfarin concomitantly. Careful monitoring of prothrombin time in patients receiving coumarin type anticoagulants is recommended.Sulphonylureas: In healthy volunteers, fluconazole prolonged the serum half-life of oral sulphonylureas such as chlorpropamide, glibenclamide, glipizide and tolbutamide, when co-administered. Fluconazole and oral sulphonylureas may be concomitantly administered to diabetics but the possibility of an hypoglycaemic episode should be considered.Hydrochlorothiazide: Co-administration of fluconazole and multiple dose hydrochlorothiazide to healthy volunteers during a kinetic interaction study, increased plasma concentrations of fluconazole by 40%. However, although the prescriber should bear this in mind, the fluconazole dose in patients receiving concomitant diuretics should not need to be altered.Anti-arrhythmic drugs Cases of QTc prolongation and/or torsades de pointes may occur after concomitant use of fluconazole with Class I, Class Ia, and all Class III anti-arrhythmic agents. Even though no formal drug interaction studies have been done, concomitant use of these anti-arrhythmic agents and drugs known to prolong the QT interval is not recommended.Pharmacodynamic interactions Medicinal products that prolong QT interval: Case reports indicate that fluconazole might have the potential to induce QT prolongation leading to serious cardiac arrhythmia. Patients treated concomitantly with fluconazole and other drugs that prolong QT interval should be carefully monitored, since an additive effect cannot be excluded.Benzodiazepines: Substantial increases in midazolam concentrations and psychomotor effects are observed when oral midazolam and fluconazole (oral or intravenous) are co-administered. The effect on midazolam appears to be greater when fluconazole is administered orally than when fluconazole is administered intravenously. If concomitant administration of benzodiazepines and fluconazole is required then the prescriber should consider reducing the benzodiazepine dose and appropriate monitoring of the patient should be undertaken.Phenytoin: Levels of phenytoin may increase to a clinically significant degree during co-administration with fluconazole. Phenytoin levels should be monitored and the phenytoin dose adjusted to maintain therapeutic levels if co-administration is necessary.Carbamazepine, Phenobarbital, Phenytoin, Rifapentine Concurrent administration of fluconazole and carbamazepine, phenobarbital, phenytoin or rifapentine results in enhanced metabolism of fluconazole, potentially reducing fluconazole effective inhibitory serum concentrations. An increase in the fluconazole dose should be considered in patients receiving concomitant carbamazepine, phenobarbital, phenytoin or rifapentine.Oral Contraceptives: Studies on the use of combined oral contraceptives with multiple doses of fluconazole have been performed. No relevant effects on hormone levels occurred during a study with fluconazole 50mg, whilst the AUCs of ethinylestradiol and levonorgestrel were increased by 40% and 24% respectively during a study with fluconazole 200mg. It is therefore considered that multiple dose fluconazole is unlikely to affect the efficacy of the combined oral contraceptive.Rifampicin: A 25% decrease in the AUC and 20% shorter half-life of fluconazole occurred when fluconazole and rifampicin were administered concomitantly. An increase in the fluconazole dose should be considered in patients receiving concomitant rifampicin.Endogenous Steroid: No effect on endogenous steroid levels was observed in females when treated with fluconazole 50mg daily. No significant effect on endogenous steroid levels or on ACTH stimulated response was observed in healthy male volunteers when treated with fluconazole 200 to 400mg daily.Ergot derivatives Because of the potential for serious toxicity including vasospasm that can occur with increased plasma concentrations of ergot derivatives (dihydroergotamine, ergoloid mesylates, ergonovine, ergotamine, methylergonovine, methysergide) the concurrent use of fluconazole and ergot derivatives is contraindicated (see Contraindications). Fluconazole and ergot derivatives are both metabolized by cytochrome P450 3A4 enzymes, and the competition for metabolism could result in a rapid onset of increased plasma concentration of the ergot derivative. Losartan CYP2C9 and CYP3A4 are involved in the metabolism of losartan to its active carboxylic acid metabolite E-3174 that is responsible for most of the angiotensin II receptor antagonism that occurs with losartan therapy. Fluconazole was shown to significantly inhibit the conversion of losartan to this metabolite. Monitoring of patients for continued control of their hypertension is recommended.Ciclosporin: In a kinetic study it was found that fluconazole 200mg daily slowly increases ciclosporin concentrations in renal transplant patients, but a multiple dose study with fluconazole 100mg daily showed no effect on ciclosporin levels in patients with bone marrow transplants. It is therefore recommended that ciclosporin plasma concentration is monitored in patients receiving fluconazole.Fentanyl, Methadone Coadministration of fluconazole may cause decreased clearance of fentanyl or methadone and subsequent increased or prolonged opioid effects (CNS depression, respiratory depression). Dosage adjustment of the opioid may be necessary.Theophylline: Use of fluconazole 200mg for 14 days showed an 18% decrease in the mean plasma clearance of theophylline. Patients who require high doses of theophylline or who may be at increased risk of theophylline toxicity should be monitored for signs of theophylline toxicity when fluconazole is co-administered. The therapy should be modified if signs of toxicity occur.Terfenadine: Fluconazole 200mg daily did not show a prolongation in the QTc interval. Use of fluconazole (taken in multiple doses of 400mg and 800mg per day) and terfenadine concomitantly, significantly increased plasma levels of terfenadine. Spontaneous reports of palpitations, tachycardia, dizziness and chest pains have occurred in patients taking fluconazole and terfenadine concomitantly where the relationship of the reported adverse events to drug therapy or underlying medical condition is uncertain. It is recommended that terfenadine and fluconazole should not be administered concomitantly due to the potential seriousness of such an interaction. (See 4.3).Cisapride: Cardiac events including torsades de pointes have been reported in patients receiving fluconazole and cisapride concomitantly. Most of these patients appear to have been predisposed to arrhythmias or had serious underlying medical conditions, and the relationship of the reported events to a possible drug interaction is uncertain. Co-administration of cisapride is contra-indicated in patients receiving fluconazole. (See 4.3).Zidovudine: Zidovudine levels in AIDS or ARC patients were determined before and after daily treatment with fluconazole 200mg for 15 days. A significant increase in zidovudine AUC was observed (20%). A second study in HIV infected patients also showed a significant increase in zidovudine AUC (74%) when fluconazole was administered concomitantly. Patients received zidovudine 200mg every eight hours either with or without fluconazole 400mg for seven days on two occasions, 21 days apart. The increased zidovudine levels are most likely caused by a decrease in the conversion of zidovudine to its major metabolite. It is recommended that patients receiving fluconazole and zidovudine be monitored for zidovudine related adverse reactions.Rifabutin: Increased serum levels of rifabutin have been reported in patients receiving fluconazole and rifabutin concomitantly, suggesting a possible interaction. Uveitis has also been reported in patients receiving this combination. It is therefore recommended that patients are carefully monitored.Tacrolimus, Sirolimus: Increased serum levels of tacrolimus have been reported in patients receiving fluconazole and tacrolimus concomitantly, suggesting a possible interaction. There have also been reports of nephrotoxicity in patients receiving this combination. Patients receiving tacrolimus or sirolimus and fluconazole concomitantly should be carefully monitored.Astemizole or other drugs metabolised by the cytochrome P450 system taken concomitantly with fluconazole may be associated with elevations in serum levels of these drugs in patients. Fluconazole should be co-administered with caution in these circumstances and careful monitoring of patients should be undertaken.Studies show that when fluconazole is taken orally with food, cimetidine, antacids or following total body irradiation for bone marrow transplantation, the absorption of fluconazole is not significantly impaired. Drug drug interaction studies with other medications have not been conducted but prescribers should be aware that such interactions may occur. | |