<p>Diabetes is the leading cause of chronic kidney disease. Hyperglycaemia in diabetes promotes the generation of reactive oxygen species, leading to diabetic complications. Although numerous antioxidant agents have been tested, none are currently recommended for the prevention of diabetic nephropathy. The Kampo formula Tsudosan, a traditional Japanese herbal medicine, possesses potent antioxidant properties. Therefore, its efficacy against diabetic nephropathy was investigated in diabetic Wistar rats. Vitamin E served as a conventional antioxidant, and dapagliflozin, a sodium–glucose cotransporter 2 inhibitor, as a representative antiglycaemic. After 25 weeks of continuous drug administration, renal biomarkers and glomerular structural changes were assessed. At week 25, proteinuria was significantly ameliorated in the Tsudosan, dapagliflozin, vitamin E, and Saireito groups. Glomerular filtration rate was significantly lower in the dapagliflozin and diabetic control groups. The glomerular staining area for 8-hydroxy-2′-deoxyguanosine, a marker of oxidative stress, was significantly reduced in the Tsudosan group, to a level comparable with controls. Urinary concentrations of liver-type fatty acid-binding protein, a marker of tubular injury, were significantly reduced in the Tsudosan and vitamin E groups. The overlapping area ratios of podocin and CD2AP staining in the glomeruli were significantly greater in all the drug-treated and control groups than those in the diabetic group. In conclusion, the antioxidative Kampo formula, Tsudosan, demonstrated efficacy in preventing diabetic nephropathy in rats, showing superior performance compared with that of vitamin E. Specifically, Tsudosan may be most beneficial when administered in the early stages of diabetes.</p> Graphical abstract <p></p>