Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
FLAGYL ER vs CHOLAC
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Metronidazole, a nitroimidazole antibiotic, undergoes intracellular reduction by bacterial nitroreductases, forming cytotoxic compounds that damage DNA and inhibit nucleic acid synthesis, selectively targeting anaerobic bacteria and protozoa.
Lactulose is a synthetic disaccharide that is not absorbed in the small intestine. It is metabolized by colonic bacteria to short-chain fatty acids, primarily lactic acid and acetic acid, which lower the colonic p H. This acidification traps ammonia (NH3) as ammonium (NH4+) in the gut lumen, reducing serum ammonia levels. Additionally, the osmotic effect of lactulose draws water into the colon, producing a laxative effect.
Treatment of bacterial vaginosis (FDA-approved),Off-label: Clostridium difficile infection, anaerobic infections, trichomoniasis, amebiasis, giardiasis, rosacea, periodontal disease, Helicobacter pylori eradication
Treatment of hepatic encephalopathy (portal-systemic encephalopathy) in patients with acute and chronic liver disease,Constipation (including chronic idiopathic constipation)
750 mg orally once daily for 10 days for bacterial vaginosis.
15-30 m L (10-20 g lactulose) orally once daily, titrated to produce 2-3 soft stools per day; maximum dose 60 m L/day. For hepatic encephalopathy: 30-45 m L (20-30 g) orally 3-4 times daily, titrated to 2-3 soft stools per day.
Terminal elimination half-life: 6-8 hours (increased to 10-12 hours with hepatic impairment; unchanged in renal impairment).
0.5-1.5 hours for lactulose; active metabolites (e.g., acetic acid) have negligible systemic half-life due to rapid local metabolism.
Hepatic metabolism via side-chain oxidation and glucuronidation; metabolites are 5-nitroimidazoles and hydroxy metabolites; CYP450 enzymes (CYP2A6, CYP3A4, CYP2B6) partially involved.
Not absorbed systemically. Metabolized by colonic bacteria (e.g., Lactobacillus, Bacteroides) to lactic acid, acetic acid, and other short-chain fatty acids.
Renal: 60-80% (metabolites and unchanged drug). Fecal: 6-15%. Minimal biliary.
Primarily fecal (biliary excretion of unchanged drug and metabolites); minimal renal excretion (<5%).
<20% (albumin).
Negligible (<1%); not significantly bound to plasma proteins.
0.5-0.8 L/kg; indicates extensive tissue distribution including CNS.
Approximately 0.2 L/kg; indicates distribution primarily in extracellular fluid.
Oral: 80-95% (extended-release formulation).
Oral: <2% systemic bioavailability due to extensive first-pass metabolism and local gut action; rectal: minimal systemic absorption.
No adjustment necessary for GFR >10 m L/min; for GFR <10 m L/min, consider using immediate-release metronidazole instead of FLAGYL ER due to lack of data in severe renal impairment.
No dose adjustment required for renal impairment.
Child-Pugh Class A/B: no adjustment necessary. Child-Pugh Class C: reduce dose to 375 mg orally once daily (50% of usual dose).
No specific Child-Pugh based adjustments. Use with caution in severe hepatic impairment due to risk of electrolyte disturbances; monitor serum electrolytes.
Safety and efficacy not established for FLAGYL ER in pediatric patients. Use immediate-release metronidazole for pediatric dosing.
Infants: 2.5-10 m L/day in divided doses. Children: 40-90 mg/kg/day (as lactulose) divided 1-2 times daily, titrated to produce soft stools. For hepatic encephalopathy: 2.5-10 m L (1.7-6.7 g) orally 3-4 times daily, titrated to 2-3 soft stools per day.
No specific dose adjustment recommended based on age alone; use caution due to potential for decreased renal function and monitor for adverse effects.
Initiate at lower end of dosing range (15 m L once daily) and titrate slowly to avoid diarrhea and electrolyte imbalance; monitor renal function and electrolytes.
Carcinogenicity: Metronidazole has been shown to be carcinogenic in mice and rats. Avoid chronic use. Reserved for anaerobic and protozoal infections.
No FDA black box warning.
Peripheral neuropathy (risk with prolonged use), CNS effects (seizures, encephalopathy), disulfiram-like reaction with alcohol, sodium overload (each tablet contains 84 mg sodium), hepatic impairment may increase risk of toxicity, renal impairment (dose adjustment not typically required but monitor), superinfection including C. difficile diarrhea.
Electrolyte disturbances (e.g., hypernatremia) may occur, especially with prolonged use or in patients with renal impairment,Diarrhea can lead to fluid and electrolyte loss; dosage should be adjusted to produce 2-3 soft stools per day,Galactose content: lactulose contains galactose and lactose; use with caution in patients with galactose intolerance, Lapp lactase deficiency, or glucose-galactose malabsorption,Risk of colonic perforation in patients with severe colonic ulceration, toxic megacolon, or gastrointestinal obstruction
Hypersensitivity to metronidazole or other nitroimidazoles; concurrent use of disulfiram (psychotic reactions); caution in pregnancy (first trimester only if clearly needed; crosses placenta); breastfeeding (use caution due to potential carcinogenicity).
Patients with galactosemia (due to galactose content),Gastrointestinal obstruction (including ileus),Hypersensitivity to lactulose or any component of the formulation
Avoid alcohol and any products containing alcohol (e.g., mouthwash, cough syrups, cooking wine) during therapy and for 48 hours after last dose. No specific food restrictions otherwise.
No specific food restrictions. Mixing with fruit juice, water, or milk may improve taste. Avoid excessive intake of dairy products if lactose intolerant (lactulose may contain small amounts of lactose).
Trimester 1: Crosses placenta; contraindicated in first trimester due to risk of carcinogenicity in animal studies and potential teratogenicity; use only for life-threatening infections. Trimester 2 and 3: Use with caution; associated with increased risk of cleft lip/palate in some studies; avoid if possible.
Lactulose is not absorbed systemically; no teratogenic effects reported in animal studies or human case reports. FDA Pregnancy Category B. Trimester-specific risks: no known fetal harm in any trimester.
Excreted in breast milk; M/P ratio ~0.9; American Academy of Pediatrics considers compatible with breastfeeding, but advise caution; monitor infant for diarrhea or oral thrush.
Excretion into breast milk is negligible due to minimal systemic absorption. M/P ratio not determined. Considered compatible with breastfeeding.
No specific dose adjustments recommended based on pregnancy pharmacokinetics; however, due to increased GFR in pregnancy, consider monitoring therapeutic levels for severe infections.
No dose adjustment required during pregnancy; pharmacokinetics unchanged due to localized GI action.
FLAGYL ER (metronidazole extended-release) is indicated for bacterial vaginosis. Avoid alcohol during therapy and for 48 hours after completion due to disulfiram-like reaction. Monitor for peripheral neuropathy; discontinue if signs occur. Use with caution in hepatic impairment; dose adjustment may be needed. May cause metallic taste.
Cholac (lactulose) is used for constipation and hepatic encephalopathy. Monitor for diarrhea and electrolyte imbalances. In hepatic encephalopathy, titrate dose to achieve 2-3 soft stools per day. Syrup can be mixed with fruit juice or water to improve palatability. Onset of action is 24-48 hours for constipation; for encephalopathy, effects may take several days.
Take this medication exactly as prescribed; do not crush or chew the extended-release tablets.,Avoid all alcohol and alcohol-containing products during treatment and for 48 hours after the last dose to prevent severe nausea, vomiting, and flushing.,Complete the full course even if symptoms improve to ensure infection is fully treated.,Report any numbness, tingling, or pain in hands or feet to your doctor immediately.,Inform your healthcare provider if you have liver disease, a history of blood disorders, or are pregnant or breastfeeding.
Take exactly as prescribed. Do not change dose without consulting your doctor.,For constipation, effects may take up to 48 hours. Do not use other laxatives unless advised.,For liver disease, it helps reduce ammonia levels. Aim for 2-3 soft bowel movements daily.,May cause gas, bloating, or stomach cramps, which usually decrease over time.,Contact doctor if you have severe diarrhea, vomiting, or signs of dehydration.,Store at room temperature, away from heat and direct light.
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 FLAGYL ER vs CHOLAC, answered by our medical review team.
FLAGYL ER is a Nitroimidazole Antibiotic that works by Metronidazole, a nitroimidazole antibiotic, undergoes intracellular reduction by bacterial nitroreductases, forming cytotoxic compounds that damage DNA and inhibit nucleic acid synthesis, selectively targeting anaerobic bacteria and protozoa.. CHOLAC is a Laxative that works by Lactulose is a synthetic disaccharide that is not absorbed in the small intestine. It is metabolized by colonic bacteria to short-chain fatty acids, primarily lactic acid and acetic acid, which lower the colonic p H. This acidification traps ammonia (NH3) as ammonium (NH4+) in the gut lumen, reducing serum ammonia levels. Additionally, the osmotic effect of lactulose draws water into the colon, producing a laxative effect.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between FLAGYL ER and CHOLAC depend on the specific clinical indication. These are agents from distinct pharmacological classes and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of FLAGYL ER is: 750 mg orally once daily for 10 days for bacterial vaginosis.. The standard adult dose of CHOLAC is: 15-30 m L (10-20 g lactulose) orally once daily, titrated to produce 2-3 soft stools per day; maximum dose 60 m L/day. For hepatic encephalopathy: 30-45 m L (20-30 g) orally 3-4 times daily, titrated to 2-3 soft stools per day.. 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 FLAGYL ER and CHOLAC 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. FLAGYL ER is classified as Category C. Trimester 1: Crosses placenta; contraindicated in first trimester due to risk of carcinogenicity in animal studies and potential teratogenicity; use only for life-threatening infec. CHOLAC is classified as Category C. Lactulose is not absorbed systemically; no teratogenic effects reported in animal studies or human case reports. FDA Pregnancy Category B. Trimester-specific risks: no known fetal . Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.