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Constipation can occur in both cats and dogs, particularly as they get older. Constipation occurs when defecation becomes difficult and is of reduced frequency, or is absent (obstipation). When feces stay in the intestines longer than is necessary, too much moisture is absorbed from the stools, causing them to become dry and hard. This makes the stools difficult to expel. As a result, your dog or cat will strain to defecate and may not have a bowel movement for several days. If this condition goes untreated, the lower bowels may eventually become severely and irreversibly stretched, causing them to lose their ability to expel feces. This condition is known as "megacolon".

There are several possible reasons for constipation. Diet appears to play a significant role. For example, when a cat or dog swallows foreign materials such as hair, bones, garbage, cloth or rocks, it can lead to constipation. Prolonged lack of exercise, a change in surroundings, or a change in daily routine (i.e., stress) can lead to constipation problems as well. In these cases, pets may become reluctant to relieve themselves and become constipated. Some medical problems, such as infected anal glands or a fractured hip, can cause painful defecation and result in constipation. Some pets may have an intestinal obstruction or a nerve or muscle disorder. Older pets have less efficient gut motility which makes them prone to constipation as well. They are also usually less active and are more likely to suffer from chronic low level dehydration (especially those with compromised kidney function). Older pets are most commonly afflicted by constipation. Certain drugs can also cause constipation. Drugs such as antihistamines and motility modifiers (e.g., Immodium™) can cause the intestines to slow down, resulting in constipation. 

How can constipation be prevented? Regular grooming will prevent excessive hair ingestion and regular exercise will encourage bowel regularity. A newly formulated hairball prevention diet that is commercially available dissolves hair ingested via daily grooming and is available for your cat through your veterinarian. Access to a frequently-cleaned litter box (and in dogs, frequent opportunities to defecate) is important. Lubricant laxatives can also be effective. These usually contain a combination of mineral oil and/or petrolatum along with a flavour base. They soften and lubricate feces and thereby make it easier to expel. The use of psyllium or pumpkin or other soluble fibres is also very effective and widely used. Docusate and other emollient laxatives can be helpful where motility is reduced, as can motility modifiers (prokinetic drugs). 

As far as foods, pets should avoid bones and should have access to fresh water at all times. Dogs should be fed a moderately high fibre diet. Your veterinarian can advise you which approaches are best suited to your pet.