Berberine is a major isoquinoline alkaloid in herbs such as goldenseal, berberis, and Coptis chinensis and has been traditionally used to treat diarrhea [1]. We report here a welldocumented interaction between tacrolimus and berberine in a child with idiopathic nephrotic syndrome. A 16-year-old child was confirmed to have nephrotic syndrome after observation of 1-month history of generalized edema associated with persistent proteinuria in April 2012. The immunosuppressive therapy was initialed with prednisone (60 mg/m2/day), frequent relapses occurs when prednisone dose was reduced to 40 mg/m2 /day. Tacrolimus (0.1 mg/kg,twice daily) was then added to immunosuppressive therapy. Tacrolimus dosage adjustment was based on therapeutic drug monitoring (TDM) in order to maintain trough blood concentration (C0) in the therapeutic range of 5–15 ng/mL [2]. Blood tacrolimus concentrations were measured using an enzymemultiplied immunoassay technique (EMIT). In October 2012, tacrolimus was given at the dose of 6.5 mg twice daily. As the child developed diarrhea, berberine (0.2 g three times daily) was started. Tacrolimus C0 and increased from 8 to 22 ng*mL−1, while serum creatinine increased from 62 to 109 μmol*L−1 . Therefore, daily dose of tacrolimus was decreased to 3 mg and after 5 days, C0 was 12 ng*mL−1 , while serum creatinine decreased to 84 μmol*L−1.