Brand: Flomax Tamsulosin is in a group of drugs called alpha-adrenergic blockers. Tamsulosin relaxes your veins and arteries so that blood can more easily pass through them. It also relaxes the muscles in the prostate and bladder neck, making it easier to urinate. Tamsulosin is used to improve urination in men with benign prostatic hyperplasia (enlarged prostate). Side effects may include penis erection that is painful or lasts 4 hours or longer; severe dizziness; or fainting.