Blood in semen is almost always caused by something minor, most often an infection or inflammation in the prostate or surrounding reproductive glands. In 30 to 70% of cases, no specific cause is ever found, and the bleeding stops on its own within a few weeks. While the sight of pink or reddish semen can be alarming, it rarely signals a serious condition, especially in men under 40.
Infection and Inflammation
Infections and inflammatory conditions account for roughly 39 to 55% of all identified cases. In men under 40, a urogenital infection is the single most common explanation. This includes infections of the prostate, the epididymis (the coiled tube behind each testicle), the urethra, and the seminal vesicles, which are the small glands that produce most of your seminal fluid.
The mechanism is straightforward: when any of these structures becomes infected or inflamed, the inner lining swells and its tiny blood vessels become engorged. That swelling and increased blood flow make the vessels fragile enough to leak small amounts of blood into the semen as it passes through. Bacterial infections, including sexually transmitted infections like chlamydia, are the usual culprits, though viruses, fungi, and even parasites can sometimes be responsible. If an infection is present, you’ll often have other clues like painful urination, pelvic discomfort, or fever.
Stones, Cysts, and Blocked Ducts
Calcified deposits (essentially tiny stones) can form in the prostate, seminal vesicles, or ejaculatory ducts. Cysts can develop in these same areas. Both stones and cysts can physically block the narrow ducts that carry semen. When a duct is blocked, nearby blood vessels dilate under pressure and can rupture, releasing blood into the ejaculate. Enlarged seminal vesicles and benign prostate enlargement are also found in some men with persistent blood in their semen.
Medical Procedures
Prostate biopsy has become one of the most common triggers for blood in semen, largely because prostate cancer screening has made the procedure so widespread. Between 6 and 13% of men who have a prostate biopsy will notice bloody semen afterward, and it can persist for up to four weeks. A vasectomy can also cause it, typically lasting a week or more. These are expected side effects, not complications.
Blood Thinners and Medications
If you take anticoagulants (like warfarin) or antiplatelet drugs (like aspirin or clopidogrel), your blood doesn’t clot as readily, which makes bleeding from even minor irritation more likely to show up in your semen. This doesn’t necessarily mean the medication caused a new problem. It may simply reveal bleeding from a small, fragile vessel that would have gone unnoticed otherwise.
Systemic Health Conditions
Less commonly, blood in semen reflects something happening elsewhere in your body. Severe high blood pressure accounts for an estimated 5% of cases. The elevated pressure can damage small blood vessels throughout the body, including those in the reproductive tract. Chronic liver disease, certain blood clotting disorders like Von Willebrand disease, and lymphomas are other systemic conditions that can cause it, though these are uncommon explanations.
How Likely Is Cancer?
This is the question most people are really asking when they search for causes, and the answer is reassuring. Cancers of the prostate, bladder, or reproductive tract account for only 4 to 13% of cases overall. A large study found that among men who had blood in their semen, just 1.6% ended up needing a prostate biopsy. And in those who did have a biopsy, the cancer detection rate was actually lower (30.4%) than in men biopsied for other reasons (48%).
Age is the most important factor in assessing cancer risk. The American College of Radiology recommends more thorough investigation for men 40 and older who notice blood in their semen, or for men of any age if the bleeding is persistent, recurrent, or accompanied by other symptoms like weight loss, difficulty urinating, or pain. For younger men with a single episode and no other risk factors, monitoring alone is typically appropriate.
When No Cause Is Found
Despite the long list of possible explanations, the reality is that most cases remain unexplained. Up to 70% of men who undergo a full evaluation never receive a definitive diagnosis. The medical term for this is “idiopathic,” which simply means the cause can’t be identified. This sounds unsatisfying, but it actually carries good news: idiopathic cases are benign. In the vast majority, the bleeding has already stopped by the time the patient sees a urologist. A single episode in a man with no risk factors and no other symptoms typically resolves on its own and doesn’t require treatment or further testing.
What the Color and Timing Tell You
Fresh, bright red blood usually means the bleeding source is close to the urethra or happened recently. A brownish or darker tint suggests the blood has been sitting in the seminal vesicles or prostate for a while before being ejaculated. Neither color, on its own, tells you how serious the cause is. What matters more is the pattern: a one-time occurrence is far less concerning than blood that shows up repeatedly over weeks or months, particularly if you also notice blood in your urine, pain during ejaculation, or swelling in the groin or testicles.