<p>When 6-mercaptopurine or azathioprine is given concurrently with Allopurinol, only one-quarter of the usual dose of 6- mercaptopurine or azathioprine should be given because inhibition of xanthine oxidase will prolong their activity. Evidence suggests that the plasma half-life of vidarabine is increased in the presence of allopurinol. When the two products are used concomitantly extra vigilance is necessary, to recognise enhanced toxic effects. Theophylline levels should be monitored in patients starting or increasing allopurinol therapy. An increase in frequency of skin rash has been reported among patients receiving ampicillin or amoxicillin concurrently with allopurinol compared to patients who are not receiving both drugs. Reports suggest that the plasma concentration of ciclosporin may be increased during concomitant treatment with allopurinol.</p>