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The use of compounded formulations of chemotherapy in veterinary medicine is common. The purpose of this study was to evaluate the drug amount of two compounded chemotherapeutics (chlorambucil and cyclophosphamide) from multiple veterinary compounding pharmacies, to determine if there was a difference in drug amounts between those that came from 503A versus 503B pharmacies, and finally to determine heterogeneity in drug amounts within each individual pharmacy. Nine veterinary compounding pharmacies (eight 503A, one 503B) were sampled in total, with two different batches sampled from each pharmacy. Each capsule's actual concentration was compared to the intended (prescribed) concentration. Of the 68 total samples obtained, 20 (29%) tested outside the FDA-acceptable discrepancy of ±10%. Of these, 12 (60%) were chlorambucil and 8 (40%) were cyclophosphamide. 503A cyclophosphamide samples had an average discrepancy of 6.6% from the intended dose while samples from the 503B pharmacy had a discrepancy of 1.8%. 503A chlorambucil samples had an average discrepancy of 10.4% from the intended dose while samples from the 503B pharmacy had a discrepancy of 9.6%. Heterogeneity within the same pharmacy and batch ranged from 0.1% to 51% for the 503A pharmacies and 2.6% to 7.5% for the 503B pharmacy. Heterogeneity between different batches within the same pharmacy ranged from 0.4% to 58.3% for the 503A pharmacies and 5% to 14.8% for the 503B pharmacy. Although the drug amounts of compounded cyclophosphamide and chlorambucil manufactured by the 503B compounding pharmacy was more reliably maintained compared to that compounded by the 503A pharmacies, there was ultimately still potential for variability in drug amounts regardless of the pharmacy designation., (© 2024 John Wiley & Sons Ltd.)