The compatibility of allopurinol sodium with selected drugs during simulated Y-site injection was studied. Five-milliliter samples of allopurinol sodium 3 mg/mL in 0.9% sodium chloride injection were combined with 5-mL samples of solutions of 92 other drugs in 0.9% sodium chloride injection at clinically used concentrations at 22 degrees C (5% dextrose injection was used as the diluent for one drug, amphotericin B). Immediately after and one and four hours after the samples were combined, visual examinations were performed in fluorescent light with the unaided eye and with a high-intensity monodirectional light (Tyndall beam) to enhance the visualization of small particles and low-level turbidity. The turbidity of each drug combination was measured as well. Combinations yielding inconclusive results were subjected to particle sizing and counting. Most of the drugs tested were compatible with allopurinol sodium 3 mg/mL during the observation period. However, 34 drugs showed various incompatibilities with allopurinol sodium, including dense turbidity, particulate formation and precipitation, color change, and effervescence. Allopurinol sodium 3 mg/mL in 0.9% sodium chloride injection was compatible with 58 drugs for up to four hours at 22 degrees C; 34 drugs were not compatible with allopurinol sodium.