This is very likely to be related. Blood in semen or ejaculate is called hematospermia. This can be caused by many conditions. The most common cause of blood in semen is inflammation of the seminal vesicles or prostate gland (prostatitis). Prostatitis is part of the urinary tract infection. Macrodantin is quite specific antibiotic for the urinary tract infection, so it would be considered better than the clindamycin.
Ideally, a physical examination by a urologist (doctors who will deal it) can give clue of the site of infection or pathology. Digital rectal examination, in which physician, puts a finger in the rectum to palpate prostate can reveal tenderness in prostate or seminal vesicles in prostatitis. A urine culture or semen culture can identify the organisms and their sensitivity towards the antibiotics. Read the resource;
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