Constipation, Megacolon & Colonoscopy Prep Calculator
Weight-based PEG 3350 treatment plans for feline deobstipation, dog and cat colonoscopy bowel prep, maintenance laxative dosing and the in-house PEG electrolyte solution recipe, with a timed, printable schedule.
For veterinary professionals. This tool doses in-hospital treatments: nasogastric lavage, IV maropitant and enemas. Pet owners: if your dog or cat is straining, not passing faeces or vomiting, please see your vet.
Quick answer
For feline medical deobstipation, give PEG 3350 electrolyte solution by nasogastric tube at 6 to 10 mL/kg/hr for around 8 hours (range 5 to 24 hours), with maropitant 1 mg/kg IV once daily, after correcting dehydration and electrolytes. For colonoscopy prep, give 30 mL/kg (cats) or 60 mL/kg (dogs) via NGT 8 to 12 hours before the procedure and repeat 2 hours later, with a warm water enema after each dose and a third enema before anaesthesia.
Build the treatment plan
Choose the protocol
Four plans from the written protocol. Deobstipation is the medical alternative to anaesthetised manual evacuation.
Patient weight
Every volume in the plan is calculated from this number.
Protocol settings
The maintenance laxative is not weight-based in the written protocol. It starts at a fixed spoon dose and is titrated to stool quality, so there is nothing to set here.
Timing (optional)
Set the planned procedure time and the schedule becomes real clock times you can print.
Patient factors
Tick anything that applies. These change the warnings, not the maths.
Treatment plan
Make the PEG 3350 electrolyte solution
Professional use only. This calculator restates a written clinical protocol for veterinarians and veterinary nurses. Doses are extra-label. Verify every dose, drug strength and route against the product label and the patient before administration. It does not replace clinical judgement and is not advice for pet owners.
Protocol reference tables
These tables are the written protocol this calculator runs on; the few cells that go beyond the written protocol are marked as such. The calculator checks them against its own internal data every time the page loads, and shows a red warning if they ever drift apart.
Protocol summary table (dogs and cats)
| Protocol | Antiemetic | PEG 3350 electrolyte solution | Fasting | Enemas |
|---|---|---|---|---|
| Cat · medical deobstipation | Maropitant 1 mg/kg IV q24h | NGT at 6–10 mL/kg/hr for 8 hours (published range 5–24 hours) | Not specified in the written protocol; treat once rehydrated (beyond the written protocol) | Warm water enemas may be used alongside lavage (beyond the written protocol) |
| Cat · colonic cleansing (colonoscopy prep) | Maropitant 1 mg/kg IV q24h | 30 mL/kg via NGT 8–12 hours before the procedure, repeated 2 hours after the first dose | Fast 24–36 hours | Warm water enema after each dose; third enema just before anaesthesia |
| Dog · bowel prep (colonoscopy) | Maropitant 1 mg/kg IV q24h | 60 mL/kg via NGT 8–12 hours before the procedure, repeated 2 hours after the first dose | NPO 24–36 hours (NGT liquid nutrition or elemental diet may be used) | Warm water enema after each dose; third enema just before anaesthesia |
| Cat · maintenance laxative | Not required | Plain PEG 3350 powder (Miralax, no electrolytes): start 1/8–1/4 level teaspoon in food twice daily, titrated upwards as needed | Not applicable | Not applicable |
In-house PEG 3350 electrolyte solution recipe (Miralax + Pedialyte)
| Component | Amount | Notes |
|---|---|---|
| Plain PEG 3350 powder (Miralax) | 220 g total, divided into 3 portions of about 73 g | Plain PEG 3350 only, no stimulant additives |
| Plain Pedialyte (unflavoured) | 3 × 360 mL bottles | Plain and unflavoured only |
| Method | Add ~73 g Miralax to each 360 mL bottle and dissolve | One bottle ≈ 360 mL of finished solution |
| Yield | Approximately 1 litre (1080 mL) from 3 bottles | Scale bottle count to the patient's calculated volume |
| Constituent | Approximate concentration |
|---|---|
| PEG 3350 | 210 g/L |
| Sodium | 3.09 g/L |
| Chloride | 3.72 g/L |
| Potassium | 0.234 g/L |
Constipation, obstipation and megacolon are three different problems
Constipation is infrequent or difficult passage of dry, hard faeces. It is common, often secondary, and usually responds to rehydration, a laxative and fixing the underlying cause. Obstipation is constipation the patient can no longer overcome: the colon is impacted and the animal cannot defecate at all. Megacolon is the end stage, a persistently dilated, poorly motile colon, most often idiopathic in middle-aged cats, and it changes the long-term plan because a flaccid colon does not recover just because you empty it once.
Before reaching for this calculator, ask why the patient is constipated. The list worth walking every time: dehydration from chronic kidney disease or any systemic illness; a narrowed pelvic canal after an old pelvic fracture; orthopaedic or spinal pain that makes posturing hurt; drugs (opioids, anticholinergics, some antacids); litter box and behavioural factors in cats; perineal or anal disease; dietary indiscretion with bone or hair; and in the rare lane, dysautonomia and sacral spinal deformity in Manx cats. Emptying the colon without addressing the cause books the patient in for a repeat visit.
The two PEG 3350 products are not interchangeable
This is the single most important distinction on this page. PEG 3350 with electrolytes (GoLYTELY and equivalents, or the Miralax + Pedialyte recipe above) is an osmotically balanced lavage solution. Because it is isosmotic and carries electrolytes, large volumes pass through without causing net fluid or electrolyte shifts, which is exactly what you want when giving 30 to 60 mL/kg or running a nasogastric infusion for hours. Plain PEG 3350 (Miralax alone) has no electrolytes. In small daily doses it is an excellent maintenance laxative that holds water in the faeces. At lavage volumes it is the wrong tool: the protocol's lavage doses all specify the electrolyte solution.
Medical deobstipation, step by step
The traditional approach to the obstipated cat is general anaesthesia and manual evacuation. The medical alternative in this protocol uses a slow nasogastric infusion of PEG 3350 electrolyte solution to soften and float the impaction from above, and in most cats it works within hours, without an anaesthetic, and with less colonic trauma.
Before the infusion starts
Rehydrate first and check electrolytes. Obstipated cats are almost always dehydrated, and many are hypokalaemic; low potassium itself impairs colonic smooth muscle, so an uncorrected potassium is part of the reason the colon stopped. Rule out the contraindications: this is a treatment for colonic impaction, not for small intestinal obstruction, perforation or untreated ileus, so image the abdomen if there is any doubt about what you are treating.
Running the infusion
Place a nasogastric tube and confirm its position before anything goes down it. Give maropitant 1 mg/kg IV once daily, then infuse the PEG 3350 electrolyte solution at 6 to 10 mL/kg/hr for around 8 hours; the published range runs from 5 to 24 hours. Expect faeces, often impressively, within 6 to 12 hours. Warm water enemas can be used alongside. If nothing has moved after a full infusion, or the cat deteriorates, reassess rather than simply extending: repeat imaging, and consider anaesthetised evacuation.
Colonoscopy bowel prep
A colonoscopy is only as good as the prep, and residual faeces is the commonest reason a scope tells you nothing. The protocol structure is the same in both species; only the volume changes. Withhold food for 24 to 36 hours (in dogs, nasogastric liquid nutrition or an elemental diet may be used through the fast). Give maropitant 1 mg/kg IV once daily. Then 8 to 12 hours before the procedure give the PEG 3350 electrolyte solution by nasogastric tube, 60 mL/kg in dogs and 30 mL/kg in cats, and repeat the same dose 2 hours later. Follow each dose with a warm water enema, and give a third enema immediately before anaesthesia. The calculator turns all of that into clock times once you enter the planned procedure time.
Enemas: warm water only in cats
The protocol specifies warm water enemas; 5 to 10 mL/kg of warm water, given slowly with a lubricated soft catheter, is a commonly used volume (the written protocol does not fix one). The hard rule: never give a sodium phosphate (Fleet-type) enema to a cat or small dog. They cause rapid, life-threatening hyperphosphataemia and hypocalcaemia and have killed cats at label volumes. Soap, hydrogen peroxide and hot water have no place either. Warm water, patience, and lubrication.
Beyond this protocol
This calculator doses one written protocol; it is not the whole management of the constipated patient. Chronic cases usually also need a maintenance plan: the PEG 3350 laxative above, dietary management (either a low-residue or a fibre-enriched approach depending on the case), water intake work, litter box management, analgesia where posturing hurts, and treatment of the underlying disease. Prokinetics such as cisapride and osmotic laxatives such as lactulose are widely used alongside PEG but are not part of this protocol and are not dosed here. Cats with true refractory megacolon despite good medical management are surgical candidates for subtotal colectomy, and referral before the third or fourth deobstipation is kinder than after the sixth.
Common mistakes this page is designed to prevent
- Using plain Miralax at lavage volumes. The lavage doses specify PEG 3350 WITH electrolytes. Plain PEG without electrolytes belongs in food, a quarter teaspoon at a time.
- Not confirming the nasogastric tube. Sixty mL/kg into an airway is a disaster. Confirm placement before every dosing session.
- Skipping rehydration and potassium. Lavaging a dehydrated, hypokalaemic cat treats the X-ray, not the patient, and the colon will not move without potassium.
- Reaching for a phosphate enema in a cat. Never. Warm water only.
- No antiemetic on board. Maropitant is in the protocol because large-volume NGT lavage makes patients nauseous, and vomiting during lavage is an aspiration risk.
- Flavoured electrolyte solution. The recipe specifies plain, unflavoured Pedialyte. Flavoured versions add sweeteners and are not what the recipe was built on.
- Treating the episode and not the cause. Every deobstipation earns the patient a differential list and a maintenance plan, or you will meet again.
FAQs
What is the PEG 3350 protocol for a constipated or obstipated cat?
For medical deobstipation, this protocol gives PEG 3350 electrolyte solution by nasogastric tube at 6 to 10 mL/kg/hr for around 8 hours (published range 5 to 24 hours), with maropitant 1 mg/kg IV once daily, after dehydration and electrolyte deficits have been corrected. Most cats pass faeces within hours without needing an anaesthetic for manual deobstipation.
How much Miralax can a cat have for constipation?
The starting laxative dose in this protocol is 1/8 to 1/4 of a level teaspoon of plain PEG 3350 powder (roughly 0.7 to 1.4 g) mixed into food twice daily, titrated upwards to effect. The target is one to two soft, formed stools a day.
What is the difference between PEG 3350 with electrolytes and plain PEG 3350?
PEG 3350 with electrolytes (for example GoLYTELY) is the balanced large-volume solution used for bowel lavage and deobstipation, because it does not cause net fluid or electrolyte shifts. Plain PEG 3350 (for example Miralax) is a single agent without electrolytes, used in small daily doses as a maintenance laxative. The two are not interchangeable at lavage volumes.
How do you make PEG 3350 electrolyte solution from Miralax and Pedialyte?
The in-house recipe in this protocol divides 220 g of plain PEG 3350 powder into three portions of about 73 g and adds one portion to each 360 mL bottle of plain, unflavoured Pedialyte. Three bottles make approximately 1080 mL of PEG 3350 electrolyte solution, containing roughly 210 g/L PEG, 3.09 g/L sodium, 3.72 g/L chloride and 0.234 g/L potassium.
How do you prepare a dog for colonoscopy?
In this protocol: food is withheld for 24 to 36 hours (nasogastric liquid nutrition or an elemental diet may be used), maropitant 1 mg/kg IV is given once daily, then 60 mL/kg of PEG 3350 electrolyte solution is given by nasogastric tube 8 to 12 hours before the procedure and repeated 2 hours later. A warm water enema follows each dose and a third enema is given just before anaesthesia.
How do you prepare a cat for colonoscopy?
The same structure as the dog protocol but at 30 mL/kg per dose: fast 24 to 36 hours, maropitant 1 mg/kg IV once daily, 30 mL/kg PEG 3350 electrolyte solution by nasogastric tube 8 to 12 hours before the procedure, repeated 2 hours later, with a warm water enema after each dose and a third before anaesthesia.
Why is maropitant given before PEG bowel prep?
Large-volume nasogastric PEG administration commonly triggers nausea and vomiting. Maropitant 1 mg/kg IV once daily is built into the protocol as an antiemetic to reduce vomiting and, with it, the risk of aspiration during and after lavage.
Can you give a cat a Fleet enema?
No. Sodium phosphate (Fleet-type) enemas cause life-threatening hyperphosphataemia and hypocalcaemia in cats and small dogs and must never be used in them. This protocol uses warm water enemas.
Is this calculator for pet owners?
No. It is a clinical tool for veterinarians and veterinary nurses. Nasogastric lavage, IV maropitant and enemas are in-hospital treatments. If your pet is straining, not passing faeces or vomiting, see your vet, or talk to a vet through the ASK A VET™ app.
Sources
- Plumb's Veterinary Drug Handbook: polyethylene glycol 3350 (with and without electrolytes) and maropitant monographs.
- Washabau RJ, Day MJ. Canine and Feline Gastroenterology. Elsevier. (Constipation, obstipation and megacolon.)
- Ettinger SJ, Feldman EC, Côté E. Textbook of Veterinary Internal Medicine. (Diseases of the large intestine.)
- Carr AP, Gaunt MC. Constipation resolution with administration of polyethylene glycol solution in cats (ACVIM Forum abstract).
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