Chapter 50 Laxative Agents
Definition of Constipation
Constipation is an important pathophysiologic condition of both cats and dogs, but primarily of the cat. Constipation is defined as difficult, painful, or reduced defecation over a period of time ranging from days to weeks or months.1 Physical examination findings depend on the severity and pathogenesis of constipation. Dehydration, weight loss, abdominal pain, and mild to moderate mesenteric lymphadenopathy are common findings in cats with idiopathic constipation. It is important to consider an extensive list of differential diagnoses in an individual animal, but it should be kept in mind that most cases are idiopathic, orthopedic, or neurologic in origin (see Chapters 10 and 58).2 Left untreated or poorly monitored, constipation may progress to obstipation (permanent loss of function) and megacolon (permanent loss of form and function).3
Established Laxative Agents
Laxatives promote evacuation of the bowel through stimulation of fluid and electrolyte transport or increases in propulsive motility. Established laxative agents are characterized as bulk-forming, emollient, lubricant, hyperosmotic, or stimulant laxatives, according to their mechanism of action (Table 50-1).3 Newer laxative agents are more specific in their mechanism(s) of action, and include chloride channel activation, guanylate cyclase activation, µ-opioid receptor antagonism, 5-HT4 serotonergic receptor agonism, and neurotrophin-3 activation.5–7
Table 50-1 Classification, Examples, and Doses of Established and Newer Laxative Agents
| Drug Classification and Example | Dose |
|---|---|
| Rectal Suppositories | |
| Dioctyl sodium sulfosuccinate | 1 to 2 pediatric suppositories |
| Glycerin | 1 to 2 pediatric suppositories |
| Bisacodyl | 1 to 2 pediatric suppositories |
| Enemas | |
| Warm tap water | 5 to 10 mL/kg |
| Warm isotonic saline | 5 to 10 mL/kg |
| Dioctyl sodium sulfosuccinate | 5 to 10 ml/cat |
| Dioctyl calcium sulfosuccinate | 250 mg (12 mL) given per rectum |
| Mineral oil | 5 to 10 mL/cat |
| Lactulose | 5 to 10 mL/cat |
| Established Laxative Agents | |
| Bulk Laxatives | |
| Psyllium | 1 to 4 tsp mixed with food, every 24 or 12 hours |
| Canned pumpkin | 1 to 4 tsp mixed with food, q24h |
| Coarse wheat bran | 1 to 4 tsp mixed with food, q24h |
| Emollient Laxatives | |
| Dioctyl sodium sulfosuccinate | 50 mg PO, q24h |
| Dioctyl calcium sulfosuccinate | 50 mg PO, q12 or 24h as needed |
| Lubricant Laxatives | |
| Mineral oil | 10 to 25 mL PO, q24h |
| Petrolatum | 1 to 5 mL PO, q24h |
| Hyperosmotic Laxatives | |
| Lactulose | 1 mL per 4.5 kg PO |
| Polyethylene glycol | 25 mL/kg PO q24h as needed |
| Stimulant Laxatives | |
| Bisacodyl | 5 mg PO q24h |
| Newer Laxative Agents | |
| Chloride Channel Activators | |
| Lubiprostone | Safe and effective doses not yet established |
| Guanylate Cyclase Activators | |
| Linaclotide | Safe and effective doses not yet established |
| µ-Opioid Antagonists | |
| Methylnaltrexone | 0.15 mg/kg SQ q24 to 48h |
| Alvimopan | Safe and effective doses not yet established |
| Serotonergic 5-HT4 Agonists | |
| Prucalopride | 0.01 to 0.2 mg/kg PO q8 to 12h as needed |
| Neurotrophins | |
| Neurotrophin-3 | Safe and effective doses not yet established |
Bulk-Forming Laxatives
Most bulk-forming laxatives are dietary fiber supplements of poorly digestible polysaccharides and celluloses derived principally from cereal grains, wheat bran, and psyllium. Some constipated cats will respond to supplementation of the diet with one of these products, but many require adjunctive therapy (e.g., other types of laxatives or colonic prokinetic agents). Dietary fiber is preferable because it is well tolerated, more effective, and more physiologic than other laxatives. Fiber is classified as a bulk-forming laxative, although it has many other properties. The beneficial effects of fiber in constipation include increased fecal water content, decreased intestinal transit time, and increased frequency of defecation.8,9 Short-chain fatty acids derived from fiber fermentation in the gut have been shown to directly stimulate colonic motility.8,9 Fiber supplemented diets are available commercially, or the pet owner may wish to add psyllium (1 to 4 tsp per meal), wheat bran (1 to 2 tbsp per meal), or pumpkin (1 to 4 tbsp per meal) to canned cat food. To maximize the therapeutic effect, cats should be well hydrated before commencing fiber supplementation. Fiber supplementation is most beneficial in mildly constipated cats, prior to the development of obstipation and megacolon. In obstipated and megacolon cats, fiber may in fact be detrimental. Low-residue diets may be more beneficial in obstipated and megacolonic cats.
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