CONDITION
Constipation and Megacolon
Constipation describes difficulty passing faeces, or passing them less often than usual. Megacolon is a structural consequence: the colon becomes persistently dilated and loses much of its ability to contract effectively. In cats, megacolon often develops after prolonged or recurrent constipation; in dogs, both conditions are less common. Owners often notice straining in the litter tray or on walks, sometimes with little produced, or small, hard, dry stools appearing after several days. The posture can resemble the signs of urinary straining, and 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 obstinate constipation and restore some normality to bowel function.
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. In dogs, large breeds and older animals may be more prone, especially if pelvic injuries, prostate enlargement, or orthopaedic pain make posturing difficult.
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.
Signals & patterns
Early signals
Straining without result
The animal adopts a posture to defecate but produces little or nothing, or only small, dry pellets. This can be mistaken for urinary straining, particularly in cats, where both involve similar body language and vocalisation.
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, the colon can press on the stomach or trigger nausea. Animals may eat less, show interest in food then walk away, or vomit intermittently, sometimes bringing up food shortly after eating.
Lethargy and withdrawal
The animal may become quieter, spend more time resting, or show less interest in play or interaction. This reflects both the physical discomfort and the metabolic effects of prolonged constipation.
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.
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 manual desobstruction 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, addresses dehydration. Weight reduction in obese cats can also reduce mechanical compression and 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 and stool softeners reduce friction. Prokinetic drugs, including cisapride, may enhance colonic contractions in cases where some motility remains.
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 sedation or anaesthesia may be necessary. Enemas—using warm water, saline, or specific solutions—soften and flush out retained material. This approach is often repeated as needed.
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, though faeces typically remain softer and more frequent postoperatively.
Trade-offs: Surgery carries anaesthetic and surgical risk. Stools are commonly soft to liquid for the first few weeks afterward, but in most cats consistency firms up within one to two months, with normal bowel movement frequency and preserved continence; a minority of cats have persistently softer stools long-term. 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, or treatment of metabolic disorders.
Trade-offs: Correction of the underlying cause does not guarantee restoration of normal colonic motility, particularly if chronic stretching has already occurred. Some causes, such as old pelvic fractures, cannot be reversed, and management becomes palliative.
Common misconceptions
"If a cat is straining in the litter tray, it must be constipated."
Straining can equally indicate lower urinary tract disease, and distinguishing between the two relies on careful observation of what is produced and where the discomfort appears to be centred. Both conditions can occur in the same animal, and urinary obstruction in male cats requires a different urgency of response than constipation.
"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.
"Hairballs cause constipation."
Hairballs are usually vomited and rarely obstruct the colon. Constipation and megacolon more often result from motility problems, dehydration, or structural issues rather than ingested hair. Excessive grooming and hairball frequency can both 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.
Anal Gland Disease in Dogs
Anal gland disease can cause pain around the anus and rectum, which may lead a dog to avoid or delay defecation. This reluctance can contribute to constipation, and conversely, hard or infrequent stools may reduce the natural emptying of the anal glands.
Prostatic Hyperplasia
Prostatic hyperplasia can result in an enlarged prostate pressing on the rectum from above, narrowing the space through which faeces must pass. This mechanical effect may contribute to straining, ribbon-shaped stools, or difficulty defecating in intact male dogs.
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.
The boundary between occasional constipation and progressive megacolon is not always sharp, and the trajectory can unfold over months. Patterns in litter-tray behaviour, appetite, and weight are all part of the wider metabolic picture. Related pages on hydration, obesity, and electrolyte disturbances may add context, and tracking frequency and character of defecation over time can be a useful foundation for future conversations.
Last reviewed: 24 April 2026 · Dr Alastair Greenway MRCVS