There are many possible reasons for bloody semen. Some of the more common ones include:

• Infection. Viral, bacterial, fungal, and parasitic infections can cause blood in the semen, including some sexually-transmitted infections like gonorrhea, chlamydia, and herpes.

• Inflammation. Prostatitis (inflammation of the prostate gland) and epididymitis (inflammation the epididymis, the tube that holds sperm) are common causes of bloody semen, but inflammation of related areas can cause it, too.

• Blockage. If any of the ducts in the reproductive area are blocked, blood vessels can swell and break, leading to bleeding.

• Medical procedures. Vasectomies, prostate biopsies, radiation therapy (including brachytherapy), some enlarged prostate procedures, and injections for hemorrhoids may cause bleeding.

• Tumors. Blood in the semen can be a symptom of prostate or testicular cancer. Tumors may also be found in the epididymis or the seminal vesicles (glands that help make much of the fluid in semen). Benign polyps (tissue growths) may also cause bleeding.

Some other possible causes of bloody semen are:

• Injury to the pelvic area
• Injury from rigorous sex
• Prolonged period of not having sex
• Enlarged prostate
• Problems with blood vessels
• High blood pressure
• Hemophilia
• Leukemia
• Chronic liver disease
• HIV infection
• Tuberculosis
• Side effect of the drug warfarin

Sometimes, no direct cause can be found and the bleeding goes away on its own.