CONDITION
Constipation and Megacolon
Constipation describes difficulty passing faeces, or passing them less often than usual. Megacolon describes a colon that has become persistently dilated and has lost much of its ability to contract effectively. In cats with megacolon, the most common form is idiopathic megacolon, in which the smooth muscle of the colon wall itself appears to contract poorly, rather than megacolon simply being the end result of repeated constipation. Narrowing of the pelvic canal after a pelvic fracture has healed is another important cause, and nerve injury and dehydration contribute in some cats. Owners often notice straining in the litter tray, sometimes with little produced, or small, hard, dry stools appearing after several days. The posture is the same as that of a cat straining to pass urine. In a male cat, straining that produces no urine can be a urethral obstruction, which can cause death within 24 to 48 hours, and from outside the tray the two are often impossible to tell apart. Appetite may fall. In longer-standing cases, the abdomen may feel full, and some animals vomit or become visibly uncomfortable. This page explores what these patterns can look like, what may be happening in the colon and the systems that influence it, how the picture is investigated, and the range of approaches used to manage persistent constipation and megacolon.
Why this matters now
Constipation can appear at any age, but certain patterns emerge. Middle-aged and older cats are more often affected, particularly those that are overweight, inactive, or have long hair that contributes to hairball formation. Dehydration, whether from reduced water intake, kidney disease, or hot weather, tends to be a common thread. Pelvic fractures that have healed with a narrowed canal, spinal problems, and arthritic pain that makes crouching in the tray uncomfortable can all contribute.
The course varies widely. Some animals experience a single episode triggered by dehydration, a dietary change, or ingestion of bone or foreign material, and the problem resolves without recurrence. In others, repeated episodes become more frequent, the colon begins to stretch, and the muscle tone deteriorates. Once megacolon develops, the bowel may lose much of its ability to contract effectively, and the condition often becomes self-perpetuating. In idiopathic megacolon the colonic muscle is abnormal from the outset, so episodes tend to recur even when each one is cleared. Within a single episode the picture can change over days: the longer faeces remain, the harder and larger the mass becomes, and a cat that begins vomiting, stops eating or becomes quiet is typically dehydrated and may need fluids and removal of faeces under anaesthesia rather than dietary change.
Signals & patterns
Early signals
Straining without result
The animal adopts a posture to defecate but produces little or nothing, or only small, dry pellets. The crouched posture, repeated trips to the tray and crying look the same when a cat is straining to pass urine. In a male cat, a blocked urethra can cause death within 24 to 48 hours as waste products and potassium build up in the blood, and whether urine is being passed is often not possible to judge by watching. An absence of urine clumps in the litter, licking at the genitals, or vomiting and quietness alongside straining fit a blocked bladder at least as well as constipation.
Reduced frequency of defecation
What was once a daily routine may become every other day, or less. Owners often notice the litter tray or garden remains cleaner than usual, though the change can be gradual enough to go unnoticed at first.
Harder, drier stools
When faeces are passed, they may be noticeably firmer, darker, or fragmented. The surface can appear cracked or pebble-like, and the animal may take longer than usual to finish.
Discomfort during defecation
The animal may vocalise, shift position repeatedly, or leave the litter tray or chosen spot without finishing. Some cats begin to associate the tray with discomfort and avoid it, defecating in unusual places.
Later signals
Abdominal distension
The abdomen may appear fuller or feel firm when touched, particularly in the lower half. In long-haired animals this can be harder to see, but changes in posture or reluctance to be handled may accompany it.
Vomiting or reduced appetite
As faecal material accumulates, a heavily distended colon can trigger nausea. Cats may eat less, show interest in food then walk away, or vomit intermittently, sometimes bringing up food shortly after eating. Vomiting and refusing food are often seen once constipation has progressed to obstipation, where faeces can no longer be passed, and dehydration adds to the problem. The same signs in a cat that has been straining also occur with urethral obstruction.
Lethargy and withdrawal
The animal may become quieter, spend more time resting, or show less interest in play or interaction. This can reflect physical discomfort as well as the dehydration and electrolyte changes of prolonged obstipation. In a cat that has also been straining, lethargy is one of the signs of a blocked bladder progressing, and the two causes cannot be separated without an examination.
Click to read about the biological mechanisms
How this is usually investigated
Investigation typically begins with a detailed history, frequency and character of defecation, diet, water intake, litter-tray habits, any previous episodes, and observation of the animal's posture and behaviour. Physical examination often includes abdominal palpation to assess faecal burden and rectal examination to check for masses, strictures, or pelvic abnormalities. Further tests are selected based on the severity, duration, and whether an underlying cause is suspected. In a straining cat, one of the first things established on examination is whether the bladder is large, firm and painful, which points to urethral obstruction rather than constipation and cannot be determined from outside the litter tray.
Physical examination
Radiography
Chemistry panel
Total T4
Abdominal ultrasound
Options & trade-offs
Management is usually a combination of approaches tailored to the individual animal, the severity of impaction, and whether an underlying cause has been identified. Some animals respond well to dietary and medical adjustments, while others require repeated removal of retained faeces under anaesthesia or surgical intervention. What proves workable varies between households and depends on the cat's temperament, the owner's capacity, and how far the colonic function has deteriorated.
Dietary modification and hydration support
Increasing dietary fibre, through prescription diets, psyllium, or pumpkin, can add bulk and moisture to faeces, promoting movement through the colon. Encouraging water intake, whether through wet food, water fountains, or flavoured broths made without onion or garlic (both toxic to cats), addresses dehydration. Weight reduction in overweight cats can also improve mobility.
Trade-offs: Response is variable, and some cats find high-fibre diets unpalatable. In advanced megacolon, dietary change alone may not restore colonic motility, and excessive fibre can worsen obstruction if the colon is severely dysfunctional.
Laxatives and prokinetic agents
Osmotic laxatives such as lactulose draw water into the colon, softening faeces. Lubricant laxatives ease passage, and stool softeners help water mix into faeces. Prokinetic drugs, including cisapride, may enhance colonic contractions in cases where some motility remains. Cisapride is not licensed in the UK, having been withdrawn from human use in many countries because of heart-rhythm effects in people, so it is prescribed under the veterinary cascade and obtained through specialist supply routes.
Trade-offs: Medication often requires daily administration and dose adjustment. Prokinetics are less effective in advanced megacolon where smooth muscle function is severely impaired, and long-term laxative use can lead to electrolyte disturbances in some animals.
Manual evacuation and enemas
When faeces become impacted, manual removal under general anaesthesia, once the cat has been rehydrated, may be necessary. A tube is placed in the windpipe because cats can vomit during the procedure. Enemas using warm water, saline, or other solutions suitable for cats soften and flush out retained material, and this approach is often repeated as needed. Phosphate-containing enemas sold for human use can cause severe electrolyte disturbances in cats, including a life-threatening rise in blood phosphate, which is why they are avoided in this species.
Trade-offs: The procedure is invasive and requires veterinary visits, sometimes frequently. It does not address underlying motility problems, and recurrence is common in cats with megacolon. Repeated anaesthesia carries cumulative risk, particularly in older or systemically unwell animals.
Subtotal colectomy
Surgical removal of the majority of the colon is considered when medical management fails and quality of life is compromised by recurrent impaction. The remaining short segment of colon and rectum can still absorb some water.
Trade-offs: Surgery carries anaesthetic and surgical risk. Stools are commonly soft to liquid for the first few weeks afterwards, but in most cats consistency firms up within one to two months. Bowel movements often remain somewhat softer and more frequent than before surgery, with continence preserved. The outcome is generally favourable in carefully selected cases, but the decision involves weighing ongoing medical management burden against permanent changes in faecal consistency.
Management of underlying causes
When constipation is secondary to pelvic injury, nerve damage, hypokalaemia, hypercalcaemia, or other identifiable disease, addressing the primary problem may improve colonic function. This can include electrolyte correction, pain relief, treatment of metabolic disorders, or surgery to widen a narrowed pelvic canal.
Trade-offs: Correction of the underlying cause does not guarantee restoration of normal colonic motility, particularly if chronic stretching has already occurred. Some changes cannot be undone. Where a healed pelvic fracture has narrowed the pelvic canal, surgery to widen it is an option, and colonic function can recover when this is done while the colonic changes and the signs have been present for less than about six months. Where they have persisted longer, the colon is often permanently damaged, and subtotal colectomy is more often considered.
Common misconceptions
"If a cat is straining in the litter tray, it must be constipated."
Straining can equally indicate lower urinary tract disease, and straining to pass urine is commonly mistaken for constipation. A urethral obstruction in a male cat can cause death within 24 to 48 hours, whereas constipation usually develops over days. Feeling the abdomen shows whether the bladder is large and firm, which is not something that can be judged by watching the tray. Both conditions can occur in the same cat.
"Once megacolon develops, the colon will never work again and surgery is inevitable."
Some cats with megacolon can be managed medically for months or years with combinations of diet, laxatives, prokinetics, and periodic manual evacuation. Surgery is typically considered when medical approaches no longer maintain acceptable quality of life, not as an automatic next step.
"Constipation in a cat is simply a hairball problem."
Swallowed hair can contribute to constipation, particularly in long-haired cats, and hairballs are recognised as one cause. They are rarely the whole explanation for recurrent or severe constipation, which more often involves idiopathic megacolon, a narrowed pelvic canal, dehydration or other disease. Excessive grooming and frequent hairballs can themselves be signs of other underlying problems.
Related conditions
Colitis
Colitis describes inflammation of the large intestine, the same structure that becomes stretched and dysfunctional in megacolon. Straining to pass faeces can occur in both conditions, and chronic colitis may contribute to motility changes that predispose to constipation over time.
Rectal Prolapse
Rectal prolapse can develop as a consequence of prolonged or severe straining, which may occur in animals with constipation or megacolon. The two conditions share straining as a visible sign, though the underlying cause and tissue involvement differ.
Hypothyroidism in Cats
Hypothyroidism slows many metabolic processes, including the muscular contractions that move waste through the colon. In cats with low thyroid hormone, reduced gut motility may contribute to constipation, though the condition itself is uncommon in this species.
Urethral Obstruction
Urethral obstruction produces the same crouched, straining posture in the litter tray as constipation, most often in male cats. A blocked cat can die within 24 to 48 hours as waste products and potassium build up, so the two are separated by examination rather than by watching the tray.
Feline Idiopathic Cystitis
Feline idiopathic cystitis causes repeated, painful attempts to pass small amounts of urine, which are commonly mistaken for straining to pass faeces. It is also one of the causes of urethral obstruction in male cats.
Chronic Kidney Disease in Cats
Chronic kidney disease leaves many cats persistently dehydrated, and the colon draws more water from faeces in response, making them harder and drier. It is one of the recognised contributors to constipation in older cats.
Straining in the litter tray covers two very different time courses. Constipation usually builds over days, and megacolon over months or years, whereas a urethral obstruction in a male cat can cause death within 24 to 48 hours. The two look alike from outside the tray, and whether the bladder is blocked is established by feeling the abdomen rather than by watching. Within constipation itself, vomiting, refusing food or quietness, together with no faeces for two to three days, can indicate obstipation, where dehydration develops and retained faeces become harder to clear the longer they stay. Once these acute pictures have been accounted for, patterns in litter-tray behaviour, appetite, hydration and weight over time can inform longer-term conversations about diet, medication and, in some cats, surgery.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS