Polypharmacy Beyond Prescriptions: A Case of Acute Interstitial Nephritis and Hyponatremia Induced by a Spanish Herbal Supplement

Introduction: The dietary supplement market is rapidly expanding in polypharmacy, with over 90,000 products available in the U.S., many lacking solid pharmacodynamic data. Herbal remedies, weight-loss products, and bodybuilding supplements have been linked to acute interstitial nephritis (AIN), but the mechanisms are unclear. These supplements are often overlooked in medication reviews, complicating related diagnoses. Case Description: A 58-year-old female with no significant medical history presented with severe symptomatic hyponatremia (Na 110 mmol/L), along with muscle tremors and insomnia. She had recently taken a single dose of Trimethoprim-sulfamethoxazole and intermittently used azole antifungals, loperamide, and a Spanish supplement (Feminabiane CU Flash) marketed for urinary tract symptoms. Her husband reported poor oral intake, dehydration, and sleep deprivation due to upcoming travel. Laboratory findings included CK ~10,000 U/L, rising creatinine from baseline 0.6 to 1.4 mg/dL, transaminitis, and a urinalysis with sterile pyuria, granular casts, and ketonuria. The patient was managed for overcorrection of sodium and referred for nephrology evaluation. Kidney biopsy revealed focal tubular injury with mild interstitial inflammation, favoring a diagnosis of medication-induced AIN. She was started on prednisone 60 mg daily, with Trimethoprim-sulfamethoxazole and famotidine for prophylaxis. Sodium stabilized, and she was discharged with close outpatient nephrology follow-up for steroid tapering. Neurology referral was also arranged due to concern for underlying Huntington’s disease. Discussion: This case highlights the diagnostic challenge of poorly regulated supplements with complex ingredient profiles. Although the exact causative agent could not be determined, the timing and clinical presentation suggest the supplement contributed to AIN and electrolyte derangements. Clinicians should routinely inquire about dietary and herbal supplement use in patients presenting with unexplained AKI. Greater awareness is needed to recognize supplements as a potential and underappreciated cause of AIN.Ingredients