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If this has been an isolated occurrence, it is typically due to a rupture of a small blood vessel. We typically are not concerned about isolated occurrences of blood in the semen. If there are other symptoms present, then it may warrant evaluation for the other symptoms, but blood in the semen alone does not need evaluation.
If the blood in the semen is recurrent, then it still may be ruptured blood vessels, but there would be greater concern that there is another condition that can be causing the blood in the semen. The most common cause of blood in the semen is an infection of the prostate, although there can be infection anywhere in the male genital tract. It also can occur from benign enlargement of the prostate or stones in the prostate. Less commonly, it can be due to a growth in the urethra or prostate, some of which are benign, such as a cyst, but some of which are cancer. While cancer would obviously be the most worrisome potential cause of bleeding, it is actually found to be the cause in only a minority of men that present with recurrent blood in the semen.
So, if this has happened once, it is not a concern and no further evaluation is necessary, but if this is recurring, then it is most likely an infection, but could be due to a variety of other conditions, so it would be appropriate for you to be seen.
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