Background and Objectives: Urethral strictures are the most common complications after surgical treatments of benign prostatic hyperplasia (BPH). Despite various preventive measures, the search for medications with antiproliferative activity and the development of surgical procedures to prevent the development of urethral strictures are still relevant. We evaluated the preventive efficacy of 5-fluorouracil against urethral strictures in patients undergoing surgery for BPH. Materials and Methods: A non-randomized clinical trial including 246 male patients with an average age of 70.0 ± 8.0 years was conducted. The main study group included 124 patients who, in addition to the standard treatment, received lavage with a 5-fluorouracil solution (1000 mg/20 mL per 500 mL of 0.9% isotonic saline) using a modified three-way urethral catheter. The monitoring of clinical, laboratory, and instrumental parameters was carried out 10 days, 3 months, and 6 months after surgery. Results: The evaluation of severity for dysuria symptoms in patients using the IPSS scale throughout the entire follow-up period showed a statistically significant decrease in ischuria and stranguria, prolongation of the interval between urinations, a decrease in intermittent urination, urinary incontinence, and straining before urination in the main group in comparison with the control patients. The patients of both study groups noted an improvement in the quality of life. It was found statistically significant decrease in the maximum urinary flow rate in the main group (p < 0.001). In the control group, after three months, four cases of urethral strictures and stenosis were recorded; after six months, this rate reached nine cases (7.3%), while in the main group, only one patient with infravesical obstruction was found (0.8%) (χ2 = 3.855, p < 0.05). Conclusions: The results of our study could indicate the effectiveness of the antiproliferative drug 5-fluorouracil in combination with use of a modified catheter in relation to the development of postoperative urethral strictures.