Reduction in Wearable-Derived Maximal Oxygen Consumption (VO₂max) During Tamsulosin Use in an Endurance Athlete

α₁-adrenergic antagonists such as tamsulosin are widely prescribed for lower urinary tract symptoms (LUTS) secondary to benign prostatic hyperplasia. While their systemic vascular effects are well recognized, their impact on aerobic performance remains unclear. We describe a case of a competitive endurance athlete who observed a decline in wearable-derived maximal oxygen consumption (VO₂max) temporally associated with tamsulosin therapy. A male endurance runner with LUTS was initiated on treatment with tamsulosin. Subsequently, he noted an increased perceived level of exertion during training and a reduction in estimated VO₂max during activity as recorded by his smartwatch. Longitudinal running data from his device were reviewed across three consecutive time periods: prior to initiation of tamsulosin (from August 15, 2022, to March 1, 2023), during daily tamsulosin therapy (from March 1, 2023, to February 7, 2024), and following transition to daily tadalafil therapy (from February 7, 2024, to June 15, 2024). A total of 113 outdoor running sessions were included, demonstrating a decline in estimated VO₂max from baseline pre-tamsulosin (48.19 mL/kg/minute) to daily tamsulosin use (45.68 mL/kg/minute) (p = 6.42 × 10^-11), with partial recovery after initiation of tadalafil (46.21 mL/kg/minute) (p = 6.77 × 10^-5). This pattern was also observed among runs performed at similar paces. Although causality cannot be established from a single observation, this case suggests that α₁-adrenergic blockade may influence aerobic performance. Further investigation in larger cohorts and controlled physiologic studies is warranted to better characterize this relationship and its potential impact on exercise capacity.