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Background: Little is known about the change in drug level and the need for dose adjustment of calcineurin inhibitor when it is used with rifabutin in solid organ transplant (SOT) recipients. We aimed to analyze whether the drug level of tacrolimus significantly reduced after the use of rifabutin and to assess optimal adjustment of tacrolimus dose in SOT recipients. Methods: Of the SOT recipients in a tertiary referral center in South Korea in 2000–2019, 50 patients who maintained an unchanged dose of tacrolimus after the use of rifabutin for treating mycobacterial disease were enrolled. Their medical records were reviewed retrospectively. Results: The mean age of the patients was 53.9 ± 11.5 years. The most commonly transplanted organ was the liver (66.0%). The most common indication of rifabutin use was for treating active tuberculosis (78.0%). After rifabutin initiation, the trough level of tacrolimus decreased significantly to the subtherapeutic range in 38 (76.0%) patients. The drug levels of these 38 patients dropped from 7.2 to 3.8 ng/ml (p <.001) after rifabutin treatment. In these patients, the median 1.5‐fold increase in the tacrolimus dose was required to restore the drug level to the within‐therapeutic range. Conclusions: These findings indicate that careful tacrolimus drug‐level monitoring and dose adjustment are necessary for most SOT recipients when rifabutin is administered for the treatment of mycobacterial disease. [ABSTRACT FROM AUTHOR]