Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
COLYTE vs CONSTILAC
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Colyte is a polyethylene glycol (PEG)-based osmotic laxative that induces diarrhea by retaining water in the gastrointestinal tract via osmotic forces, thereby cleansing the colon.
Constilac (lactulose) is a synthetic disaccharide that is not absorbed in the small intestine. In the colon, it is metabolized by bacteria to short-chain fatty acids (e.g., lactic acid), which osmotically draw water into the bowel, producing a laxative effect. Additionally, in hepatic encephalopathy, the acidification of colonic contents traps ammonia (NH3) as ammonium (NH4+), reducing its absorption.
Bowel preparation prior to colonoscopy,Bowel preparation prior to barium enema,Bowel preparation prior to colorectal surgery
Constipation (FDA-approved),Portal-systemic encephalopathy (hepatic encephalopathy) (FDA-approved)
4 L oral solution administered as a single dose at a rate of 240 m L every 10 minutes until complete.
Oral, 15-45 m L (10-20 g lactulose) once daily, titrated to produce 2-3 soft stools daily; may be given as a single dose or divided twice daily. Onset of action 24-48 hours.
Not applicable; systemic absorption is negligible (<0.06%), so a terminal elimination half-life is clinically irrelevant. The gastrointestinal transit time for the solution is approximately 1-3 hours.
Not applicable due to negligible systemic absorption; lactulose acts locally in the colon.
Polyethylene glycol is not significantly metabolized and is excreted largely unchanged in feces.
Lactulose is not absorbed in the small intestine. It undergoes bacterial metabolism in the colon to lactic acid, acetic acid, and other short-chain fatty acids.
COLYTE (polyethylene glycol 3350 and electrolytes) is minimally absorbed; <0.1% of the dose is excreted renally. The majority is eliminated unchanged in feces via the gastrointestinal tract, with fecal excretion accounting for >99%.
Primarily fecal as unchanged drug (systemic absorption negligible). Renal excretion accounts for <2% of any absorbed fraction. Biliary excretion is minimal.
Not applicable; negligible systemic absorption, so protein binding is clinically irrelevant.
Negligible (<1%) due to minimal systemic absorption.
Not applicable; negligible systemic absorption, so volume of distribution is clinically irrelevant.
Not applicable (negligible systemic distribution).
Oral: <0.1% (systemic bioavailability is negligible due to minimal absorption of polyethylene glycol).
Oral: <2% (largely unabsorbed; metabolized by colonic bacteria).
No dose adjustment required for renal impairment; use with caution in severe renal insufficiency (Cr Cl <30 m L/min) due to potential electrolyte imbalance.
No dose adjustment required for renal impairment. Lactulose is minimally absorbed and renally excreted unchanged; safety established in all stages of chronic kidney disease.
No specific dose adjustments for hepatic impairment; use with caution in severe hepatic disease.
Contraindicated in Child-Pugh class C cirrhosis due to risk of hepatic encephalopathy exacerbation from colonic acidification. In Child-Pugh A and B, standard dosing may be used with caution; monitor for electrolyte disturbances and volume status.
Pediatric patients (≥6 months): 25-40 m L/kg/hour orally or via nasogastric tube until rectal effluent is clear; maximum 4 L.
Infants: 2.5-10 m L/day in divided doses. Children 1-5 years: 5-15 m L/day. Children 6-12 years: 15-30 m L/day. Adolescents: 15-45 m L/day. Administer orally; adjust to produce soft stools.
No specific dose adjustment; monitor for dehydration and electrolyte disturbances due to reduced renal reserve.
Initiate at lower end of adult dose (15 m L once daily) and titrate slowly to avoid diarrhea and electrolyte imbalances. Monitor for dehydration, hypokalemia, and hypernatremia, especially in patients with renal impairment or on diuretics.
None
None
Risk of electrolyte disturbances (especially in patients with renal impairment or those taking medications affecting electrolytes), aspiration risk (use with caution in patients with impaired gag reflex or at risk of regurgitation), serious fluid and electrolyte abnormalities, cardiac arrhythmias, seizures, and serious adverse reactions including ischemic colitis and ulcerative colitis. Use with caution in patients with severe ulcerative colitis, toxic megacolon, or gastrointestinal obstruction.
Use with caution in patients with diabetes mellitus (lactulose may contain small amounts of galactose and lactose),Electrolyte disturbances (particularly hypokalemia and hypernatremia) may occur with prolonged use,Risk of galactosemia in patients with galactose intolerance,May cause abdominal discomfort, flatulence, and diarrhea
Gastrointestinal obstruction, bowel perforation, toxic megacolon, gastric retention, ileus, known hypersensitivity to any component of the product.
Patients with galactosemia,Intestinal obstruction or suspected obstruction,Undiagnosed abdominal pain
Avoid all solid foods during bowel preparation; only clear liquids (e.g., water, clear broth, apple juice, black coffee, clear soda) are permitted. Dairy products, red or purple liquids (which can mimic blood), and alcohol should be avoided. Resume a normal diet only after the procedure.
No significant food interactions. May be mixed with juice or water to mask taste. Avoid excessive intake of dairy products if lactose intolerant, as lactulose is derived from lactose.
Category C. No adequate and well-controlled studies in pregnant women. Animal studies have not been conducted. Should be used during pregnancy only if clearly needed. Potential for fetal harm due to maternal dehydration or electrolyte imbalance.
No known teratogenic effects. Lactulose is not absorbed systemically; no fetal risk expected in any trimester.
Not known if excreted in human milk. M/P ratio not determined. Caution advised due to potential for diarrhea in nursing infant. Use only if clearly needed.
Considered safe during breastfeeding. M/P ratio not applicable as lactulose is not absorbed; no significant excretion into breast milk expected.
No specific dose adjustments recommended. Pharmacokinetic changes in pregnancy not studied; standard bowel preparation dosing should be used with caution due to increased risk of fluid and electrolyte shifts.
No dose adjustment required. Pharmacokinetics unchanged due to lack of systemic absorption.
Colyte (PEG-3350 with electrolytes) is used for bowel cleansing prior to colonoscopy. Ensure adequate hydration to prevent electrolyte imbalances. Administer in divided doses; split-dose regimen improves tolerability and cleansing quality. Contraindicated in GI obstruction, gastric retention, bowel perforation, toxic colitis, or megacolon. Monitor for bloating, nausea, and vomiting; slow rate if symptoms occur.
CONSTILAC (lactulose) is a non-absorbable disaccharide that acts as an osmotic laxative. It is also used off-label for hepatic encephalopathy by reducing serum ammonia. Onset of action for constipation is 24-48 hours. Overuse can cause electrolyte disturbances, particularly hypernatremia and hypokalemia. In hepatic encephalopathy, titrate to 2-3 soft stools per day.
Follow the prescribed dosing schedule exactly; do not skip doses.,Drink the entire solution as directed, typically with a split-dose regimen (half the evening before, half the morning of the procedure).,Stay well-hydrated; drink clear liquids after starting the preparation.,Avoid solid foods; only clear liquids are allowed until after the procedure.,Expect frequent, watery bowel movements; this is necessary for cleansing.,Notify your doctor if you experience severe bloating, vomiting, or signs of dehydration.,Do not take other medications within 1 hour of starting the preparation.
Take exactly as prescribed; effects may take 1-2 days.,May cause flatulence and cramping initially.,Mix with fruit juice or water to improve taste.,If symptoms persist beyond 1 week, consult your doctor.,Do not use if you have galactosemia or bowel obstruction.
No interactions on record
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about COLYTE vs CONSTILAC, answered by our medical review team.
COLYTE is a Osmotic Laxative that works by Colyte is a polyethylene glycol (PEG)-based osmotic laxative that induces diarrhea by retaining water in the gastrointestinal tract via osmotic forces, thereby cleansing the colon.. CONSTILAC is a Osmotic Laxative that works by Constilac (lactulose) is a synthetic disaccharide that is not absorbed in the small intestine. In the colon, it is metabolized by bacteria to short-chain fatty acids (e.g., lactic acid), which osmotically draw water into the bowel, producing a laxative effect. Additionally, in hepatic encephalopathy, the acidification of colonic contents traps ammonia (NH3) as ammonium (NH4+), reducing its absorption.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between COLYTE and CONSTILAC depend on the specific clinical indication. These are both Osmotic Laxative agents and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of COLYTE is: 4 L oral solution administered as a single dose at a rate of 240 m L every 10 minutes until complete.. The standard adult dose of CONSTILAC is: Oral, 15-45 m L (10-20 g lactulose) once daily, titrated to produce 2-3 soft stools daily; may be given as a single dose or divided twice daily. Onset of action 24-48 hours.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
No direct drug-drug interaction has been formally documented between COLYTE and CONSTILAC in current clinical databases. However, individual patient risk factors including other medications, organ function, and comorbidities should always be evaluated by a qualified healthcare provider.
The maternal-fetal safety profiles differ. COLYTE is classified as Category C. Category C. No adequate and well-controlled studies in pregnant women. Animal studies have not been conducted. Should be used during pregnancy only if clearly needed. Potential for. CONSTILAC is classified as Category C. No known teratogenic effects. Lactulose is not absorbed systemically; no fetal risk expected in any trimester.. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.