Blood in ejaculation is most commonly caused by the rupture of the delicate blood vessels in the prostate gland area or urethra.
Blood thinning medications like aspirin, ibuprofen can cause blood leakage from them commonly.
Also sexual act itself can put pressure on these blood vessels.
Enlarged prostate is another common cause of blood in semen.
Infection of he prostate gland is another possibility which also causes pain and discomfort.
Prostate cancer also can present like this.
So you should see your doctor for a clinical evaluation.
But the chances of this being harmless is very high.
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