Purinase Drug Interactions

Ampicillin/Amoxicillin: Increased frequency of skin rash occurred among patients concurrently taking allopurinol with ampicillin or amoxicillin compared to those taking allopurinol, amoxicillin, or ampicillin alone. The cause of the interaction is not yet established.
It is recommended that the combination be avoided, and alternative antibiotics be administered in patients receiving allopurinol.
Angiotensin-converting-enzyme (ACE) inhibitors (e.g., captopril): Concomitant use of captopril and allopurinol may predispose to hypersensitivity reactions such as Stevens-Johnson syndrome. Patients on the combination should be monitored and if a reaction occurs, the use of these drugs should be discontinued.
Antacids: Antacids (e.g., aluminum hydroxide) may decrease the blood levels and reduce the therapeutic effects of allopurinol when taken concomitantly.
Anticoagulants (e.g., warfarin and other coumarin derivatives): Allopurinol may prolong the elimination half-life of coumarin, but there is no evidence that this has any clinical significance. However, careful monitoring is advised for all patients taking anticoagulants.
Chlorpropamide: Allopurinol or its metabolites may compete with chlorpropamide for excretion in the renal tubule. Concomitant administration of these drugs in patients with poor renal function may increase the risk of prolonged hypoglycemic effect of chlorpropamide.
Cyclophosphamide and other cytotoxic agents (e.g., doxorubicin, bleomycin, procarbazine, mechloroethamine, bendamustine): In the presence of allopurinol, enhanced bone marrow suppression by cyclophosphamide and other cytotoxic agents has been reported among patients with neoplastic disease other than leukemia. However, allopurinol did not appear to increase the toxic reaction of these cytotoxic agents in a well-controlled study of patients treated with cyclophosphamide, doxorubicin, bleomycin, procarbazine, and/or mechloroethamine (mustine hydrochloride).
Ciclosporin: Allopurinol may increase the plasma concentration of ciclosporin.
The risk of enhanced ciclosporin toxicity should be considered during the coadministration of these drugs. If these drugs are coadministered, ciclosporin blood levels should be monitored, and possible adjustments of cyclosporin dosage should be considered.
Didanosine: The mean peak plasma concentrations (Cmax) and AUC values of didanosine were approximately doubled with concomitant allopurinol treatment (300 mg daily) without affecting its terminal half-life in healthy volunteers and in patients with human immunodeficiency virus (HIV). Concomitant use of these drugs is generally not recommended. If concomitant use is unavoidable, a dose reduction of didanosine may be required, and patients should be closely monitored.
Diuretics: Thiazide diuretics and ethacrynic acid may elevate serum oxipurinol concentrations, resulting to increased risk of serious allopurinol toxicity, including hypersensitivity, particularly in patients with renal impairment. Renal function of patients concurrently taking these medications should be monitored, even in the absence of renal failure. If reduced renal function is detected in these patients, dosage levels should be adjusted conservatively.
There have been reports of increased serum urate and plasma oxipurinol following the co-administration of allopurinol and furosemide.
Drugs that increase serum urate concentrations (e.g., pyrazinamide, diazoxide, alcohol, mecylamine): If drugs that increase the serum urate concentrations are coadministered, increase in allopurinol dosage may be required.
Mercaptopurine or azathioprine: Allopurinol inhibits the oxidative metabolism of azathioprine and mercapturine by xanthine oxidase, resulting to the prolonged activity of these drugs. If allopurinol is used concomitantly, only one-third or one-fourth of the usual doses of azathioprine or mercapturine should be initially administered; doses of azathioprine or mercapturine should be adjusted according to the patient's response and toxic effects.
Phenytoin: Hepatic oxidation of phenytoin may be inhibited by allopurinol, but the clinical significance of this interaction has not been determined.
Theophylline: Inhibition of theophylline metabolism has been reported with relatively high doses of allopurinol (i.e., 300 mg twice a day). The mechanism of the interaction may be explained by xanthine oxidase being involved in the biotransformation of theophylline in man. Theophylline levels should be monitored in patients starting or increasing allopurinol therapy.
Uricosurics and Salicylates: Drugs with uricosuric activity such as probenecid or large doses of salicylate may accelerate the excretion of oxipurinol; this may result in decreased therapeutic activity of allopurinol.
Vidarabine (Adenine Arabinoside): The plasma half-life of vidarabine is increased in the presence of allopurinol. If used concomitantly, extra caution is necessary to determine enhanced toxic effects.