Fluorescent Yellow Urine Induced by Vonoprazan Fumarate: A Case Report of a Benign Phenomenon

An abnormal color in urine may be caused by various factors, including foods with high levels of vitamins, certain medications that metabolize in the body to produce fluorescent substances, or certain metabolic disorders. Although abnormal urine, such as black, green, pink, or purple urine, is commonly reported, fluorescent urine is rare and typically observed under ultraviolet light. This case reports a healthy participant with persistent fluorescent yellow urine after receiving vonoprazan fumarate (TAK-438) under visible lights. A 25-year-old healthy female participated in a Phase I clinical trial of vonoprazan fumarate. After the oral dose of vonoprazan fumarate, her urine showed intensely fluorescent yellow in urine collection intervals under visible light. During the whole study, she had a standard diet and did not experience any adverse event. Her urinalysis showed color in yellow or mild yellow, normal pH and density, normal urobilinogen, and absence of bilirubin. This fluorescent phenomenon resolved spontaneously after drug washout. This case presents a benign, drug-induced effect in urine color, which is likely caused by vonoprazan fumarate and its metabolites. It is essential for clinicians to be aware of this effect to avoid unnecessary invasive diagnostic procedures or patient anxiety.