A 47-year-old woman developed severe kidney dysfunction after taking a red yeast rice supplement for about 7 months. She experienced sudden nausea and had an elevated serum creatinine level of 4.26 mg/dL. A kidney biopsy showed findings consistent with acute tubular necrosis. Kidney dysfunction improved after discontinuation of the supplement and corticosteroid therapy. Similar cases have been reported, raising concerns about the safety of red yeast rice supplements in Japan. An investigation into the nephrotoxic ingredients in the product is ongoing. This case highlights the need for public awareness about the risks of unregulated supplements. Red yeast rice is widely used as a food coloring, traditional Chinese medicine, and dietary supplement, especially in Asia. By April 17, 2024, 236 cases of health hazards requiring hospitalization, including serious kidney dysfunction, suspected to be related to red yeast rice supplements had been identified in Japan, leading to an urgent recall of the supplements. This report describes a case of severe kidney dysfunction possibly associated with red yeast rice supplements. The patient, diagnosed with dyslipidemia, started taking the supplement "Red Yeast Rice Cholestehelp" at her own discretion, taking 0–3 tablets per day for about 9 months. Before starting the supplement, her serum creatinine level was 1.09 mg/dL, with normal urinalysis findings. Five days prior to admission, she visited her doctor because of nausea. From that day onward, she stopped taking the supplement. Blood tests revealed an elevated serum creatinine level of 4.26 mg/dL, and urinary tests showed proteinuria, granular casts, tubular epithelial cells, and glycosuria. She had no recent history of medications, except for the supplement, prior to the onset of acute kidney injury (AKI). On admission, blood pressure was 144/88 mmHg, and body temperature was 36.6°C. Physical examination findings were uneventful. Laboratory findings on admission are shown in Figure 1. Kidney biopsy findings suggested acute tubular necrosis. Corticosteroid therapy was administered, and kidney function improved over time. The patient had mild kidney dysfunction prior to taking the supplement. Considering that kidney dysfunction was evident only in a minority of patients who took the supplement, the underlying chronic kidney disease may have increased susceptibility to AKI in this condition.A 47-year-old woman developed severe kidney dysfunction after taking a red yeast rice supplement for about 7 months. She experienced sudden nausea and had an elevated serum creatinine level of 4.26 mg/dL. A kidney biopsy showed findings consistent with acute tubular necrosis. Kidney dysfunction improved after discontinuation of the supplement and corticosteroid therapy. Similar cases have been reported, raising concerns about the safety of red yeast rice supplements in Japan. An investigation into the nephrotoxic ingredients in the product is ongoing. This case highlights the need for public awareness about the risks of unregulated supplements. Red yeast rice is widely used as a food coloring, traditional Chinese medicine, and dietary supplement, especially in Asia. By April 17, 2024, 236 cases of health hazards requiring hospitalization, including serious kidney dysfunction, suspected to be related to red yeast rice supplements had been identified in Japan, leading to an urgent recall of the supplements. This report describes a case of severe kidney dysfunction possibly associated with red yeast rice supplements. The patient, diagnosed with dyslipidemia, started taking the supplement "Red Yeast Rice Cholestehelp" at her own discretion, taking 0–3 tablets per day for about 9 months. Before starting the supplement, her serum creatinine level was 1.09 mg/dL, with normal urinalysis findings. Five days prior to admission, she visited her doctor because of nausea. From that day onward, she stopped taking the supplement. Blood tests revealed an elevated serum creatinine level of 4.26 mg/dL, and urinary tests showed proteinuria, granular casts, tubular epithelial cells, and glycosuria. She had no recent history of medications, except for the supplement, prior to the onset of acute kidney injury (AKI). On admission, blood pressure was 144/88 mmHg, and body temperature was 36.6°C. Physical examination findings were uneventful. Laboratory findings on admission are shown in Figure 1. Kidney biopsy findings suggested acute tubular necrosis. Corticosteroid therapy was administered, and kidney function improved over time. The patient had mild kidney dysfunction prior to taking the supplement. Considering that kidney dysfunction was evident only in a minority of patients who took the supplement, the underlying chronic kidney disease may have increased susceptibility to AKI in this condition.