The most common cause of an enlarged prostate in dogs is benign prostatic hyperplasia (BPH), a hormone-driven condition that affects roughly 50% of intact male dogs by age 5 and 95% by age 9. But BPH isn’t the only cause. Bacterial infections, cysts, tumors, and estrogen-producing testicular tumors can all make a dog’s prostate swell. Understanding which cause is at play matters because the treatments are very different.
BPH: The Hormonal Cause Behind Most Cases
BPH is overwhelmingly the most common reason for prostate enlargement in dogs. In one large study, it accounted for about 57% of all prostatic disease diagnoses. The condition is driven by dihydrotestosterone (DHT), a potent form of testosterone that the prostate tissue converts from regular testosterone. As a dog ages, his prostate becomes increasingly efficient at producing DHT. Research from Johns Hopkins found a near-perfect correlation (r = 0.95) between prostate size and the gland’s capacity to generate DHT.
Estrogen plays a supporting role. As intact males get older, the balance between testosterone and estrogen shifts, and estrogen appears to amplify the prostate’s ability to accumulate DHT. This is why the condition is progressive and virtually guaranteed in older intact dogs. The prostate simply keeps growing, slowly compressing the urethra and the colon that sits just above it.
Because BPH depends entirely on hormones from the testicles, neutering eliminates the testosterone supply and causes the prostate to shrink. For dogs that need to remain intact (breeding males, for example), medical options exist. A hormone-blocking implant (deslorelin) can shrink the prostate by up to 60% but must be replaced every 6 to 12 months and temporarily halts fertility. Another medication (finasteride) blocks the enzyme that converts testosterone into DHT, reducing prostate size over about 16 weeks while preserving fertility and sex drive. The catch is that the dog needs to stay on it for life, or the prostate grows back.
Bacterial Prostatitis
Bacteria can infect the prostate and cause it to swell, sometimes dramatically. The infection typically reaches the gland by traveling up the urethra from the urinary tract. E. coli is the most frequently reported culprit, followed by Staphylococcus species, Streptococcus, Proteus mirabilis, and Klebsiella. In many cases, bacterial prostatitis develops as a secondary problem in a dog whose prostate is already enlarged from BPH, because the abnormal tissue creates pockets where bacteria can establish themselves.
Acute bacterial prostatitis tends to cause obvious illness: fever, pain, reluctance to move, and sometimes bloody or cloudy urine. Chronic prostatitis can be subtler, with recurring urinary tract infections as the main clue. The distinction matters because chronic infections are harder to clear, since antibiotics have difficulty penetrating deep into prostatic tissue over long periods.
Prostatic Cancer
Prostate cancer (prostatic adenocarcinoma) is less common than BPH, accounting for about 13% of prostatic disease cases in one major study, but it’s far more serious. Unlike BPH, prostate cancer is not prevented by neutering. In fact, the evidence points in the opposite direction: castrated dogs had more than four times the risk of developing prostatic cancer compared to intact males (odds ratio of 4.34). Researchers believe neutering doesn’t initiate the cancer but may favor its progression once cells begin to change.
This creates a genuine paradox. Neutering protects against the most common prostatic problem (BPH) while potentially increasing risk for the most dangerous one (cancer). The interval between neutering and cancer diagnosis varies widely, so there’s no simple timeline to watch for. Prostate cancer in dogs tends to be aggressive and is often diagnosed late because early symptoms (straining to urinate or defecate, blood in urine) look identical to BPH.
Prostatic Cysts
Two types of cysts can enlarge the prostate or the area around it. Retention cysts form inside the prostate itself, often as a complication of BPH. As the gland’s architecture changes, small fluid-filled pockets develop within the tissue. These are generally small.
Paraprostatic cysts are a different story. These form outside the prostate from embryological remnants of female reproductive tissue that all male dogs carry. They’re lined with epithelium and fill with fluid, sometimes growing quite large before causing symptoms. Because they sit adjacent to the prostate rather than inside it, they enlarge by pressing on the bladder or colon. A dog with a paraprostatic cyst may strain during bowel movements or urination for weeks before the underlying cause is identified, since the symptoms mimic other prostatic diseases.
Estrogen-Producing Tumors
A less common but important cause of prostate changes is excess estrogen from a Sertoli cell tumor in the testicle. These tumors produce abnormally high levels of estrogen, which transforms the normal glandular lining of the prostate into a layered, skin-like tissue (a process called squamous metaplasia). The cells shed keratin and debris into the gland’s ducts, causing blockages and swelling. Dogs with Sertoli cell tumors often show other signs of estrogen excess, such as hair loss, darkened skin, or attraction from other male dogs. Removing the affected testicle resolves the estrogen source and allows the prostate to return to normal over time.
How an Enlarged Prostate Affects Your Dog
Regardless of the underlying cause, an enlarged prostate creates problems by pressing on the structures around it. The urethra runs directly through the gland, so swelling narrows the channel. You may notice your dog taking longer to urinate, producing a thin stream, or straining visibly. Complete blockage is rare but possible.
The colon sits just above the prostate, and significant enlargement compresses it. This produces ribbon-like or flattened stools and visible straining during bowel movements. Many owners initially assume their dog is constipated. Bloody urine can appear with several prostatic conditions, from BPH to infection to cancer. If the prostate is painful, as with acute infection, dogs may arch their backs, walk stiffly, or refuse to move altogether.
How the Cause Is Identified
Because the symptoms of different prostatic diseases overlap so heavily, your vet will need more than a physical exam to pin down the cause. A rectal exam gives an initial sense of size, shape, and symmetry. Ultrasound reveals internal structure: a uniformly enlarged gland suggests BPH, while irregular masses or asymmetry raise concern for cancer or abscesses.
A blood test measuring canine prostate-specific esterase (CPSE) can help gauge BPH severity. Studies show CPSE levels correlate strongly with both clinical severity and the complexity of symptoms, making it useful for tracking the condition over time. For suspected infections, fluid samples from the prostate are cultured to identify the specific bacteria involved. When cancer is suspected, a tissue biopsy or fine-needle aspirate is typically needed for a definitive answer, since imaging alone can’t reliably distinguish aggressive tumors from abscesses or cysts.