Can Urine Pass Through the Anus? Causes & Symptoms

Urine can pass through the anus, but only when something has gone wrong. In a healthy body, the urinary tract and the digestive system are completely sealed off from each other. When urine does exit through the rectum or anus, it almost always means an abnormal tunnel called a fistula has formed between the bladder or urethra and part of the bowel. These connections can develop from disease, injury, radiation, surgery, or a birth defect, and they bring a set of distinctive symptoms that often lead people to seek medical help.

How Urine Ends Up in the Wrong Place

The bladder sits just in front of the rectum, separated by layers of fat and tissue. When that barrier breaks down, a passage opens between the two organs. If the connection runs between the colon and the bladder, it is called a colovesical fistula. If it links the rectum to the urethra, it is a rectourethral fistula. Other variations exist depending on exactly which segments of the bowel and urinary tract are involved, but the result is broadly the same: contents from one system leak into the other. Urine can flow into the bowel and come out through the anus, or stool and gas can travel in the opposite direction and appear in the urine.

These fistulas develop because of congenital abnormalities, traumatic injuries, or underlying diseases such as inflammation or cancer.¹ Diverticular disease is the single most common cause, responsible for up to about 80 percent of cases, with colorectal cancer accounting for up to roughly 20 percent.²

Diverticular Disease and Crohn’s Disease

Diverticulitis, the inflammation of small pouches that bulge out from the colon wall, is by far the leading culprit. When one of those pouches becomes infected and inflamed, the resulting abscess can erode through the tissue separating the colon from the bladder. A colovesical fistula is a recognized complication of acute diverticulitis, and it can occasionally appear during the very first symptomatic episode rather than after years of repeated flare-ups.³ People with diverticular disease may not realize their condition can do more than cause belly pain and changes in bowel habits; when urine starts appearing where it should not, many are caught off guard.

Crohn’s disease is another major inflammatory cause. This chronic condition can affect any part of the digestive tract and has a tendency to bore through tissue layers. When Crohn’s-related inflammation reaches the bladder, the result is an enterovesical fistula. The hallmark symptoms include recurrent urinary tract infections, fecaluria (stool particles in the urine), and pneumaturia (gas bubbles in the urine).⁴ Crohn’s-related fistulas often require a combination of surgery and ongoing medical management of the underlying disease.

Cancer and Radiation Therapy

Cancers of the colon, rectum, bladder, or prostate can invade adjacent organs and create fistulous tracts. Tumors do not respect anatomical boundaries; as a mass grows, it can dissolve the tissue wall between the bowel and the urinary system. But cancer treatment itself can also be the trigger. Radiation therapy aimed at pelvic tumors, especially prostate cancer, damages not just the tumor but the healthy tissue surrounding it. Over time, that radiation damage can weaken the tissue enough for a fistula to form between the rectum and the urethra or bladder. One surgical center reviewing 16 years of experience documented fistulas forming between the rectum and the urinary tract in patients who had been treated with radiation for prostate cancer, calling them a dreaded complication with devastating consequences.⁵

The timeline for radiation-related fistulas is important to understand. They do not necessarily appear during treatment or even shortly after. Some develop months or years later, long after the patient considers their cancer treatment finished. Radiation weakens tissue progressively, and a seemingly minor additional insult, like a biopsy or a urinary procedure, can push already-compromised tissue past its breaking point.

Surgical Complications and Congenital Fistulas

Prostate surgery is one of the more common surgical causes. Rectourethral fistulas can form after radical prostatectomy, though they are relatively rare.⁶ Other pelvic surgeries, including procedures on the bladder, rectum, or reproductive organs, carry a small but real risk of creating unintended connections between the urinary and digestive tracts. Traumatic injuries to the pelvis, whether from accidents or penetrating wounds, can do the same.

Some people are born with these connections. Congenital rectourethral fistulas occur in newborns with certain types of imperforate anus, a condition where the anal opening does not form normally.⁷ These are typically identified and surgically repaired early in life, though the repair can carry long-term implications for bowel and bladder control. In adults born with anorectal malformations that included a rectourethral fistula, quality-of-life scores are generally comparable to the broader population, though there are signs of lower emotional well-being in some men and a negative effect on sexual life in some women.⁸

Recognizing the Symptoms

The symptoms of a fistula between the bowel and the urinary tract depend on the direction of flow. When bowel contents leak into the bladder, the most distinctive signs are pneumaturia and fecaluria. Pneumaturia means you see air bubbles in your urine or hear a hissing sound when you urinate. Fecaluria means you notice particulate matter or a brownish discoloration in your urine. Both are alarming enough that most people seek medical help quickly. Recurrent urinary tract infections are another classic sign, because bacteria from the intestine gain direct access to the normally sterile urinary tract.⁹

When urine flows in the other direction, into the bowel and out through the anus, the symptoms are less immediately dramatic but still noticeable. You might pass unusually watery stool that smells distinctly of urine, or notice that your rectal output seems more liquid than it should be regardless of what you have eaten. Some people describe a persistent wetness or leakage from the anus that does not behave like normal stool. Pain in the pelvis, between the legs, or during urination or bowel movements is common with any fistula type.

These symptoms can overlap with other conditions, including plain urinary tract infections, irritable bowel syndrome, or hemorrhoids. The distinguishing factor is that fistula symptoms tend to be persistent and unusual in character. Passing gas through the urethra or seeing fecal matter in urine are essentially diagnostic of a fistula; no other common condition produces those particular findings.

How Doctors Find the Fistula

Confirming the presence and location of a fistula usually requires imaging. Contrast-enhanced CT scans are a standard first step and can reveal the tract, the organs involved, and any associated abscess. MRI offers additional detail and is particularly useful for complex or subtle fistulas. In one study examining MRI findings in 40 cases of colovesical fistulas, the most common pattern (about 71 percent of cases) showed an abscess sitting between the bowel wall and the bladder wall, while a visible track between the two organs was seen in about 15 percent. MRI correctly identified the underlying cause in roughly two-thirds of cases.¹⁰

Cystoscopy, where a camera is passed into the bladder, can sometimes visualize the opening of the fistula on the bladder side. However, this approach does not always find the tract. In one Crohn’s disease case, cystoscopy failed to identify an obvious opening even though CT had suggested a fistula between the small bowel and the bladder.⁴ When a urinary leak is suspected after bladder surgery, measuring the creatinine level in fluid collected from a surgical drain and comparing it to blood creatinine can help confirm that urine is escaping.¹¹

Treatment and Repair

The odds of a fistula healing on its own are poor. Spontaneous closure is rare, and most cases require reconstructive surgery.¹² The specific surgical approach depends on the fistula’s size, location, cause, and whether previous repairs have failed.

For many patients, the first step is diverting both the fecal and urinary streams away from the fistula to let the surrounding tissue calm down. This typically means a temporary colostomy or ileostomy to reroute stool, and sometimes a catheter or cystostomy to divert urine. Once the area has had time to heal and any infection or inflammation has cleared, the definitive repair is performed. In a series of rectourethral fistula repairs, small and straightforward fistulas were repaired with a rectal advancement flap, where a layer of healthy rectal tissue is pulled over the opening like a patch. Larger or recurrent fistulas required interposing a piece of healthy tissue between the rectum and urethra, using a gracilis muscle flap from the inner thigh or an omental flap from the abdomen.¹³

When the fistula is caused by Crohn’s disease, surgery alone is not enough. The underlying inflammation has to be controlled with medications, or a new fistula will form. For cancer-related fistulas, treatment of the malignancy itself is part of the strategy, and the surgical plan may include removing the affected segment of bowel along with the fistula.

When Urine Is Deliberately Routed Through the Bowel

There is one situation where urine passing through the anus is not a sign of disease but of intentional surgery. Ureterosigmoidostomy is a procedure in which the ureters (the tubes that carry urine from the kidneys to the bladder) are disconnected from the bladder and implanted into the sigmoid colon. Urine then drains into the large bowel and exits through the anus along with stool. This procedure has been used historically for people born with bladder exstrophy, a condition where the bladder forms outside the body, and for some patients who have had their bladders removed due to cancer.¹⁴

Living with urine mixed into the fecal stream comes with real trade-offs. The colon reabsorbs substances differently than the bladder does. Prolonged contact between urine and bowel lining can cause electrolyte imbalances, most commonly a type of metabolic acidosis where the blood becomes too acidic and chloride levels rise. Less frequent but still possible problems include low potassium, low calcium, and low magnesium.¹⁵ There is also a long-recognized risk of bowel tumors developing at the site where the ureter meets the colon, sometimes decades after the original surgery.¹⁶ Because of these complications, ureterosigmoidostomy has largely fallen out of favor in countries where alternative urinary diversions, such as ileal conduits that drain into an external bag, are available. It still sees use in some settings where those alternatives are not practical.

Skin Problems From Urine and Stool Contact

Whether urine reaches the anal area through a fistula, a surgical diversion, or simple incontinence, the skin around the anus and perineum pays a price. Prolonged and repeated exposure to urine or stool causes a condition called incontinence-associated dermatitis, an irritant reaction characterized by redness, pain, softening and breakdown of the skin, scaling, and often secondary infection.¹⁷ The alkaline pH of urine and the digestive enzymes in stool work together to strip away the skin’s protective barrier. Once that barrier is compromised, bacteria and fungi move in quickly.

Managing this skin breakdown involves keeping the area as clean and dry as possible, using barrier creams to protect intact skin, and treating any infection that develops. For people living with a fistula while awaiting surgical repair, or for those with a ureterosigmoidostomy, diligent perineal skin care becomes a daily necessity. The irritation ranges from mild discomfort to severe enough to affect sleep, mobility, and willingness to leave the house.¹⁸

Why Some Animals Do This Normally

If the idea of urine and stool sharing an exit sounds bizarre from a human perspective, it is worth noting that most vertebrates on the planet do exactly that. Birds, reptiles, and amphibians all have a cloaca, a single chamber that receives output from the digestive, urinary, and reproductive systems. In reptiles and birds, the cloaca and rectum play an active role in regulating the body’s water and salt balance, a job that in mammals falls primarily to the kidneys.¹⁹ Mammals evolved a fully separate urinary outflow path, which offers certain advantages in infection control and waste management but means that any accidental reconnection between the two systems is pathological rather than normal physiology.

This evolutionary context helps explain why fistulas between the bowel and urinary tract cause so many problems in humans. Our urinary tract is designed to be sterile, and our colon is teeming with bacteria. Merging the two streams introduces bacteria where they do not belong and exposes the bowel lining to the chemical properties of urine it was never meant to handle. The complications of ureterosigmoidostomy, including metabolic imbalances and tumor risk, reflect the fact that the human colon is not built for double duty the way a reptile’s cloaca is.