CONDITION
Constipation and Megacolon in Cats
Constipation describes difficulty passing faeces, or passing faeces less often than usual. In cats, this may appear as straining in the litter tray, producing small or hard stools, or none at all. Megacolon is a related condition in which the large bowel becomes stretched and loses its ability to move material forward effectively, often developing after repeated or prolonged episodes of constipation. Owners typically notice a cat spending longer than usual in the litter tray, sometimes vocalising or appearing uncomfortable, or they may find no faeces in the tray for a day or more. In some cases, a cat may strain in a way that looks the same as straining to pass urine, and a blocked urethra is the more dangerous of the two possibilities. The severity of constipation ranges widely, from a single mild episode that resolves on its own to a persistent pattern that requires investigation. This page explores what constipation and megacolon can look like in practice, the range of underlying factors that may contribute, how the problem is investigated, and the spectrum of approaches used to manage it. The aim is to help you understand what may be happening and what questions might be useful to consider.
Why this matters now
Constipation is a common problem in cats, particularly affecting middle-aged to older individuals, and many cats have one or two episodes without further recurrence. The condition may begin as episodic constipation that initially responds to dietary modification or occasional medical intervention, but in some cats progresses to a state of chronic, recurrent obstipation where the colon becomes permanently dilated and loses its ability to generate effective peristaltic contractions. Certain breeds, particularly Manx and other tailless or short-tailed breeds, may have a predisposition related to sacral spinal abnormalities that affect colonic innervation. Male cats appear to be affected more frequently than females, though the condition can develop in any individual.
The progression from episodic constipation to established megacolon typically occurs over months to years, though the rate varies considerably between individuals. Early episodes of constipation may respond well to dietary fibre supplementation, increased water intake, and occasional laxative use. As the condition progresses, episodes may become more frequent, require more aggressive intervention to resolve, and leave shorter intervals of normal defecation between episodes. With each episode of significant faecal retention and colonic distension, the colon is thought to lose more of its contractile capacity. Eventually, the colon may become so dilated and atonic that it can no longer move faecal material through effectively, establishing the state of megacolon. At this advanced stage, the condition becomes refractory to medical management alone, and some cats may require surgical intervention to maintain quality of life.
Signals & patterns
Early signals
Occasional straining in the litter tray
The cat may spend longer than usual in the litter tray, adopting the posture for defecation with visible effort but producing smaller, harder stools than normal. These episodes may be intermittent, separated by periods of apparently normal defecation. The same posture is seen in a cat that cannot pass urine.
Slightly reduced stool frequency
Rather than keeping to its usual pattern of bowel movements, the cat may begin to skip days between defecations. In some cats the stools produced after these longer intervals are larger and firmer than usual.
Small, hard faecal pellets
The character of the stools may change before the frequency decreases, with normal-sized, well-formed stools being replaced by smaller, harder, drier pellets that reflect prolonged water absorption in the colon.
Occasional vomiting
Some cats may vomit intermittently during episodes of constipation. The vomiting may not at first be connected to the defecation difficulty.
Later signals
Prolonged absence of bowel movements
The cat may go for days without producing any stool, despite frequent visits to the litter tray. Productive defecation becomes increasingly rare, and when stools are passed, they may be very hard, dry, and small. Repeated visits to the tray with nothing produced are also seen in cats that cannot pass urine.
Visible straining and discomfort
The cat may cry or vocalise while attempting to defecate, spend extended periods in the litter tray, and may begin to associate the litter tray with discomfort, potentially leading to elimination outside the tray. Crying while straining is also reported in cats that cannot pass urine.
Palpable abdominal mass
In advanced cases, the retained faecal mass within the distended colon may be palpable through the abdominal wall, feeling like a firm, tubular structure in the caudal abdomen. Some owners may notice that the cat's belly feels firmer than usual.
Progressive weight loss and reduced appetite
As the condition becomes more established, the chronic discomfort and abdominal fullness may progressively reduce appetite. Weight loss may develop as food intake decreases and the cat becomes less active due to persistent discomfort.
Click to read about the biological mechanisms
How this is usually investigated
Investigation of constipation and megacolon in cats involves characterising the severity of faecal retention, assessing colonic structure, and identifying any underlying causes that may be contributing to impaired colonic function.
Physical examination and abdominal palpation
Abdominal radiography
Blood work and metabolic screening
Rectal examination
Response to evacuation
Options & trade-offs
Management of feline constipation ranges from dietary and lifestyle modifications for mild cases to surgical intervention for cats with established megacolon. The approach is typically escalated based on the severity and responsiveness of the condition.
Dietary modification and hydration
Increasing dietary moisture content through wet food, adding water to food, or encouraging water intake through fountains can help maintain faecal hydration. Dietary fibre supplementation, with either soluble fibre (such as psyllium husk, which is a food supplement and not an authorised medicine) or insoluble fibre, may help promote colonic motility and stool passage in some cats.
Trade-offs: Dietary approaches are non-invasive and may be sufficient for mild or early constipation. However, the optimal type and amount of fibre varies between individuals, and some cats with established megacolon do not respond to dietary modification alone. Excessive fibre in a cat with poor colonic motility may worsen distension rather than promote movement.
Osmotic and stool-softening laxatives
Lactulose is a commonly used osmotic laxative in cats, drawing water into the colonic lumen to soften the faecal material and stimulate motility, and a lactulose syrup is authorised in the UK for cats. Polyethylene glycol 3350 (PEG 3350, also called macrogol) is an alternative osmotic laxative. No veterinary macrogol product is authorised in the UK, so it is a human medicine used in cats under the veterinary medicines cascade. These agents help maintain faecal hydration and promote passage.
Trade-offs: Osmotic laxatives require ongoing daily administration and dose adjustment. Too little may be ineffective, while too much can cause diarrhoea, dehydration, and electrolyte disturbances. Long-term compliance requires the cat to accept the medication, which can be challenging. Lactulose may cause flatulence and abdominal cramping in some cats.
Prokinetic medications
Drugs that stimulate colonic smooth muscle contraction, such as cisapride, may help restore or augment colonic motility in cats with impaired but not completely absent muscle function. Cisapride acts on serotonin receptors in the enteric nervous system to enhance coordinated peristalsis.
Trade-offs: Prokinetics are most effective when colonic smooth muscle retains some contractile capacity. In cats with advanced megacolon whose colon no longer contracts, prokinetics may have limited benefit. Cisapride has no UK veterinary authorisation for cats and is prescribed under the veterinary medicines cascade. It is no longer used in human medicine because of heart rhythm disorders.
Manual evacuation under anaesthesia
For cats with significant faecal impaction, manual evacuation under general anaesthesia allows the colon to be emptied through a combination of enemas and digital extraction. This relieves the impaction and allows assessment of colonic recovery potential.
Trade-offs: Manual evacuation requires general anaesthesia with its associated considerations. The procedure provides temporary relief but does not address the underlying motility disorder. Some cats require repeated evacuations at intervals of weeks to months if medical management is insufficient to maintain regular defecation. Phosphate-containing enemas made for people are not used in cats, because they can cause severe electrolyte imbalances.
Subtotal colectomy
Surgical removal of the majority of the colon is considered when medical management has failed and the cat's quality of life is significantly impacted by chronic obstipation. The procedure removes the non-functional dilated colon, allowing the small intestine to connect more directly to the remaining distal colon and rectum.
Trade-offs: Subtotal colectomy is a major surgical procedure with a recovery period of several weeks. Most cats experience soft or liquid stools for weeks to months after surgery as the remaining bowel adapts, and some may have chronically soft stools long-term. However, most cats that undergo the procedure experience significant improvement in quality of life compared to their pre-surgical state of chronic obstipation. Complications can include breakdown of the join in the bowel (anastomotic dehiscence), which leads to septic peritonitis with a high death rate. Constipation recurred in 35% of patients in one study.
Common misconceptions
"Constipation in cats is always caused by insufficient fibre or water intake."
While dehydration and diet can contribute to constipation, the majority of feline megacolon cases are classified as idiopathic, meaning no specific cause is identified, and the fault appears to lie in how the colonic smooth muscle contracts and not in dietary factors. Other causes include pelvic canal narrowing from previous fractures, neurological conditions affecting colonic innervation, and metabolic disorders such as hypokalaemia or hypercalcaemia. Simply increasing fibre or water may not be sufficient if the underlying issue is colonic dysmotility.
"If a cat strains in the litter tray, it must be constipated."
Straining in the litter tray, technically termed tenesmus, can indicate either constipation (straining to defecate) or urethral obstruction (straining to urinate), and the postures adopted can appear almost identical. The two do not carry the same weight. Constipation usually builds over days. A complete urethral obstruction, which occurs almost exclusively in male cats, causes uraemia within 36 to 48 hours and death within about 72 hours, and in some cats in less than 24 to 48 hours. The reported signs of a blocked urethra are repeated attempts to urinate with little or nothing passed, crying while straining, licking around the genital area, vomiting, and refusing food. Owners are known to mistake these signs for constipation, and the posture alone does not separate the two, so the distinction often cannot be made with confidence at home. A blocked urethra is recognised on examination, where the bladder is typically hard, distended and painful.
"Once a cat develops megacolon, surgery is the only option."
Not all cats with colonic dilation require surgery. The distinction between severe constipation with recoverable colonic function and true end-stage megacolon is important. Many cats can be managed medically with a combination of dietary modification, laxatives, prokinetics, and periodic evacuation for extended periods. Surgery is typically considered when medical management no longer maintains adequate quality of life, but this threshold varies between individual cats.
Related conditions
Colitis
Colitis describes inflammation of the large intestine, the same organ affected in megacolon. In some cats, chronic colitis can contribute to straining and difficulty passing faeces, which may be mistaken for constipation or may coexist with it.
Urethral Obstruction
Urethral obstruction in male cats often presents with straining in the litter tray, a pattern that can appear very similar to the straining seen with constipation. Distinguishing between straining to urinate and straining to defecate can be difficult for owners, and the two differ sharply in timescale: a complete urethral obstruction that is not relieved leads to death within about 72 hours, and in some cats in less than 24 to 48 hours.
Anal Gland Disease
Anal gland disease can cause discomfort during defecation and may lead a cat to strain or hesitate in the litter tray. In some cases, impacted or infected anal glands contribute to reluctance to pass faeces, which can mimic or complicate constipation.
Inappropriate Elimination in Cats
Cats experiencing pain or difficulty in the litter tray due to constipation may begin to associate the tray with discomfort, sometimes leading to elimination outside the tray. This behavioural shift can persist even after the underlying physical problem has been addressed.
Rectal Prolapse
Rectal prolapse can occur as a consequence of prolonged or severe straining to defecate, particularly in young animals or those with persistent constipation or colitis. The repeated effort to pass hard or obstructed faeces may eventually cause tissue from the rectum to protrude through the anus.
Chronic Kidney Disease in Cats
Chronic kidney disease is common in older cats and causes more urine to be produced, so affected cats tend to be mildly dehydrated. The colon then draws more water from the faeces, which become dry and hard, and constipation can follow.
Monitoring litter tray output, both frequency and character of stools, is a direct way to track colonic function over time. Noting how often the cat defecates, the consistency and size of the stools produced, and whether straining is observed helps identify trends before they become critical episodes. Whether urine is still being passed is a separate matter: a straining cat that passes no urine has a different problem, one in which a complete obstruction leads to death within about 72 hours, and in some cats in less than 24 to 48 hours. Maintaining hydration through wet food, water fountains, and multiple water stations throughout the home supports colonic function. If a cat has experienced constipation episodes, understanding the pattern (how frequently they occur, what seems to trigger them, and how they respond to intervention) helps characterise the trajectory of the condition.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS