Co-administration of allopurinol (a xanthine oxidase inhibitor) with didanosine is not recommended. Co-administration of didanosine and allopurinol results in increased systemic exposure to didanosine, which can result in didanosine-associated toxicity. Patients treated with didanosine who require allopurinol administration should be switched to an alternative treatment regimen. Coadministration of allopurinol (300 mg once daily for 7 days) and didanosine buffered tablets (400 mg single dose on days 1 and 8) to healthy volunteers increased didanosine AUC and Cmax by 105% and 71%. 
Videx Summary of Product Characteristics, Bristol-Myers Squibb Pharmaceuticals Ltd, April 2016.

Coadministration is contraindicated because systemic exposures of didanosine are increased, which may increase didanosine-associated toxicity. The effect of allopurinol (300 mg/day for 7 days) on the pharmacokinetics of didanosine (400 mg single dose) was investigated in healthy volunteers (n=14). Didanosine AUC increased 1.13-fold and Cmax increased by 69%. When allopurinol (300 mg/day for 7 days) and a 200 mg single dose of didanosine was studied in 2 subjects with renal impairment, didanosine AUC increased by 312% and Cmax increased by 232%.
Videx Prescribing Information, Bristol-Myers Squibb Co, December 2018.