Docetaxel is a standard chemotherapy for metastatic prostate cancer but is associated with dose-limiting peripheral neuropathy. Currently, no pharmacologic agents are established for prevention. Tadalafil, a PDE5 inhibitor, may improve microvascular perfusion and offer neuroprotection. This randomized phase II trial evaluates whether concurrent use of tadalafil (5 mg every 2 days) reduces the incidence and severity of docetaxel-induced peripheral neuropathy compared to standard care in patients with metastatic prostate cancer.