Flomax and half marathon

Been prescribed Flomax for urine spraying / retention problem. The doctor didn't think it was absolutely necessary, so asked me to take one pill every two days. Just took the 4th pills and had some bad side effects. Almost fainted twice. I have never fainted in my life and am physically fit, can run a half-marathon without any discomfort. Other side effects include dizziness and weakness, blurry vision, stuffy nose, etc. It doesn't seem to help on my spraying / retention symptoms either. On the days I don't take Flomax I feel perfectly fine. I will stop taking this medicine as the side effects are a lot worse than the symptom it's trying to cure.