Tolterodine Dimer is primarily formed as a degradation product during the storage or manufacturing of tolterodine, an active pharmaceutical ingredient used to treat overactive bladder. While the dimer itself is not therapeutically active, its presence is critical in pharmaceutical quality control. It serves as a key indicator in stability testing of tolterodine-containing formulations. Monitoring the levels of this dimer helps ensure drug safety and efficacy by assessing the extent of degradation over time. Regulatory guidelines require strict limits on such impurities, making the dimer an important reference standard in analytical testing, particularly in HPLC and mass spectrometry methods used during drug development and quality assurance.