A swollen penis in dogs is usually caused by one of a few things: normal arousal, an infection of the sheath, the penis being stuck outside its covering (called paraphimosis), or less commonly, growths or trauma. Most of the time it’s harmless and resolves on its own, but certain signs mean your dog needs veterinary attention quickly.
Normal Swelling From the Bulbus Glandis
Dogs have a structure at the base of the penis called the bulbus glandis, which is a rounded bulge of tissue that engorges with blood during arousal. This is the most common reason owners notice sudden swelling, and it’s completely normal. It can happen during mating, humping, play, or general excitement. The two round swellings that appear near the base of the penis are sometimes mistaken for testicles, and owners of neutered dogs may even wonder whether their dog was actually neutered.
This swelling typically goes down on its own within 30 minutes or so once the dog calms down. If it resolves without any pain, discoloration, or discharge, there’s nothing to worry about. Persistent swelling or pain associated with an engorged bulbus glandis, however, is abnormal and worth investigating.
Paraphimosis: When the Penis Can’t Retract
Paraphimosis happens when the penis comes out of its sheath and can’t slide back in. The skin at the opening of the sheath folds inward and traps the exposed penis, cutting off blood flow from draining properly. The result is rapid swelling that makes retraction even harder, creating a worsening cycle. The most common triggers are breeding, excitement, trauma, or hair and debris wrapped around the shaft.
If you notice your dog’s penis is out and visibly swollen, gently check for hair, string, or any foreign material that might be constricting it. Removing that material, if present, can sometimes allow it to retract. You can also try applying a water-based lubricant to reduce friction and gently coax the tissue back into the sheath. Stay calm and handle the area carefully, as the exposed tissue becomes dry and painful quickly, and your dog may try to lick or bite at it, making things worse.
This becomes urgent when the exposed tissue turns dark red, purple, or black. Those color changes indicate the blood supply is seriously compromised, and the tissue is at risk of dying. If you can’t get the penis back in within a few minutes, or if the swelling is severe, your dog needs veterinary care right away.
Infection of the Sheath
Balanoposthitis is an infection or inflammation of the penis and its surrounding sheath, usually caused by bacterial overgrowth. The hallmark sign is discharge: a thick, yellowish-green or cloudy fluid that you might notice on your dog’s fur near the groin area or on bedding. Your dog will likely lick the area excessively and may show signs of discomfort.
A small amount of clear or slightly cloudy discharge is actually normal in sexually mature male dogs and doesn’t mean anything is wrong. It becomes a concern when the discharge is heavy, looks like pus, has a strong odor, or your dog seems bothered by it. A vet can take a swab to identify the bacteria involved and prescribe appropriate treatment, which typically clears the infection up without complications.
Prostate Issues in Intact Dogs
If your dog hasn’t been neutered, an enlarged prostate could be contributing to genital swelling or unusual discharge. Benign prostatic hyperplasia is extremely common in intact older males. The prostate gradually enlarges and can cause a range of symptoms: straining to urinate or defecate, blood dripping from the penis between urination, urinary incontinence, and a bloody or watery discharge from the sheath. In one study, straining during urination and defecation was reported in over 90% of affected dogs, while bloody discharge from the prepuce occurred in nearly 46%.
The discharge and irritation from an enlarged prostate can make the genital area look swollen or inflamed, even though the penis itself may not be the primary problem. Neutering resolves the condition in most cases because it removes the hormonal drive behind prostate growth.
Growths on the Penis
Tumors can develop on a dog’s penis, and one of the most distinctive is the canine transmissible venereal tumor (CTVT). This is a contagious cancer spread through direct contact, usually during mating. It appears as a fleshy, cauliflower-like mass on the genital tissue. These growths are fragile and bleed easily when touched. They tend to appear on the bulbus glandis area of the penis and are most common in intact dogs that have contact with strays or unscreened mates.
Other types of masses can also develop in the area. Any lump, bump, or unusual growth on or near your dog’s penis that doesn’t go away within a day or two warrants a vet visit. Diagnosis typically involves examining a tissue sample under a microscope.
Persistent Erection Without Arousal
Priapism is a persistent erection that has nothing to do with sexual excitement. It’s uncommon but serious. The two main forms involve either too much blood flowing in (often from trauma to the penis or perineum that ruptures a blood vessel) or not enough blood flowing out (a blockage in the veins). Underlying causes in dogs include spinal problems like disc herniation or lumbar stenosis, neurological disease, perineal injuries, and in rare cases, cancer spreading to the area.
If your dog has a prolonged erection that lasts well beyond any exciting stimulus, or if the penis appears swollen and your dog seems distressed, this needs professional evaluation. Prolonged engorgement without proper blood flow can permanently damage the tissue.
What to Look For
Not every instance of a swollen-looking penis is a problem. Here’s a quick way to sort through what you’re seeing:
- Swelling that resolves within 30 minutes, no pain, no discharge: likely normal arousal of the bulbus glandis.
- Yellowish or greenish discharge with licking: possible infection of the sheath, especially if the discharge is heavy or foul-smelling.
- Penis stuck outside the sheath and swelling: paraphimosis. Check for constricting hair or debris. If it won’t retract or the tissue is changing color, seek immediate care.
- Bloody discharge unrelated to urination in an intact dog: possible prostate enlargement.
- Fleshy mass or growth: needs veterinary evaluation to rule out tumors.
- Tissue turning purple or black: blood supply is compromised. This is an emergency.
The color of the exposed tissue is the single most important thing to watch. Healthy penile tissue in dogs is pink to light red. Anything progressing toward dark red, purple, or black signals that blood isn’t circulating properly, and tissue damage can become irreversible within hours.