Drug Interactions between allopurinol and guaifenesin / oxtriphylline

MONITOR: Allopurinol may inhibit the metabolism of xanthines (e.g., theophylline), possibly increasing serum concentrations and the risk of toxicity. When theophylline was coadministered with allopurinol, a 25% increase in steady-state theophylline serum concentrations were observed. This interaction may be more likely to occur with daily allopurinol doses of 600 mg or more. The mechanism may related to allopurinol's inhibition of xanthine oxidase, an enzyme involved in the metabolism of theophylline.

MANAGEMENT: Closer clinical and laboratory monitoring for adverse effects is advised if theophylline is used concomitantly with allopurinol. Additionally, theophylline levels should be more closely monitored following the initiation or discontinuation of allopurinol treatment, and the theophylline dosage adjusted as necessary in accordance with the prescribing information. Patients should be advised to report any signs of theophylline toxicity to their doctor (e.g., nausea, vomiting, diarrhea, headache, restlessness, insomnia, or irregular heartbeat).