Metronidazole

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Here is a complete, textbook-based overview of metronidazole:

Metronidazole

Class: 5-Nitroimidazole | Antibiotic / Antiprotozoal

Mechanism of Action

Metronidazole is a prodrug that requires reductive activation of its nitro group by susceptible organisms. Anaerobic and microaerophilic pathogens (e.g., T. vaginalis, E. histolytica, G. lamblia, anaerobic bacteria) contain electron transport components with a sufficiently negative redox potential to donate electrons to metronidazole. This single-electron transfer forms a highly reactive nitro radical anion that kills organisms by radical-mediated mechanisms targeting DNA.

The key enzyme involved is pyruvate-ferredoxin oxidoreductase (PFOR), which catalyzes pyruvate decarboxylation, producing electrons that reduce ferredoxin, which in turn donates electrons to metronidazole.

Importantly, O2 competes with metronidazole for electrons - this is why aerobic organisms are resistant (O2 prevents reductive activation).

Goodman & Gilman's Pharmacological Basis of Therapeutics, p. 1342

Spectrum of Activity

Protozoa:

Trichomonas vaginalis
Entamoeba histolytica (amebiasis)
Giardia lamblia

Anaerobic bacteria:

All anaerobic cocci
Anaerobic gram-negative bacilli including Bacteroides spp. (including B. fragilis)
Anaerobic spore-forming gram-positive bacilli: Clostridium spp.
Microaerophilic bacteria: Helicobacter pylori, Campylobacter spp.

NOT active against: aerobic or facultatively anaerobic bacteria; nonsporulating gram-positive bacilli are often resistant.

Pharmacokinetics (ADME)

Parameter	Details
Routes	Oral, IV, intravaginal, topical
Oral bioavailability	Essentially complete and prompt absorption
Volume of distribution	Approximates total body water
Protein binding	<20%
Half-life	~8 hours (plasma); hydroxy metabolite ~12 h
Metabolism	Hepatic - oxidation of side chains to hydroxy derivative and acid
Excretion	>75% in urine (largely as metabolites); ~10% unchanged
CSF penetration	Excellent
Other distribution	Crosses into vaginal secretions, seminal fluid, saliva, breast milk
Placenta	Crosses placenta

The hydroxy metabolite retains ~50% of the antitrichomonal activity of the parent drug. Urine may turn reddish-brown due to unidentified pigments.

Oxidative metabolism is induced by: phenobarbital, prednisone, rifampin, (possibly) ethanol. Oxidative metabolism is inhibited by: cimetidine.

Goodman & Gilman's, p. 1352-1354

Therapeutic Uses & Dosing

Trichomoniasis

Single oral dose of 2 g - cures >90% of cases
Both partners should be treated
Alternative: 500 mg twice daily x 7 days

Amebiasis (intestinal and extraintestinal)

Adults: 500-750 mg PO TID x 7-10 days
Children: 35-50 mg/kg/day in 3 divided doses x 7-10 days (max 750 mg/dose)
Drug of choice for all symptomatic forms including amebic colitis and amebic liver abscess
Must follow with a luminal amebicide (e.g., paromomycin) as E. histolytica persists in the colon

Giardiasis

Used widely though not FDA-approved; effective in practice
Tinidazole (2 g single dose) is preferred first-line

Anaerobic bacterial infections

Adults PO/IV: 30 mg/kg/24 hr divided Q6-8h; max 4 g/24 hr
IV loading dose: 15 mg/kg over 1 hour, then maintenance 6 hours later
Key infections: intra-abdominal infections, brain abscess, aspiration pneumonia, C. difficile (now second-line to vancomycin for severe CDI)

C. difficile Infection (CDI)

Vancomycin is superior to metronidazole, especially for severe CDI (vancomycin 79% vs. metronidazole 66% success rate)
Metronidazole: unlike vancomycin, is systemically absorbed so achieves colonic concentrations via systemic circulation

Helicobacter pylori Eradication

Used in bismuth quadruple therapy: PPI + amoxicillin + metronidazole + bismuth x 14 days

Bacterial Vaginosis

Oral or topical vaginal gel (0.75% or 1.3%)

Neonates (anaerobic infection)

Loading dose: 15 mg/kg x 1
Maintenance based on postmenstrual age (PMA): 7.5 mg/kg Q24h (PMA 24-25 wk) up to 7.5 mg/kg Q6h (PMA >40 wk)

Adverse Effects

System	Effect
GI	Nausea, vomiting, dry mouth, metallic taste, diarrhea, abdominal distress
CNS	Headache, dizziness, vertigo; rarely encephalopathy, convulsions, ataxia, incoordination
Peripheral nervous system	Numbness, paresthesias - drug should be withdrawn; reversal may be slow or incomplete
GU	Dysuria, cystitis, sense of pelvic pressure
Skin	Urticaria, flushing, pruritus; rarely Stevens-Johnson syndrome (more common with high doses + mebendazole)
Urine	May turn reddish-brown

Drug Interactions

Interaction	Effect
Alcohol	Disulfiram-like reaction: flushing, vomiting, abdominal distress, headache (avoid alcohol during and for 3 days after therapy)
Disulfiram	Confusional and psychotic states - contraindicated together
Warfarin	Prolongs prothrombin time - monitor INR
Lithium	May precipitate CNS signs of lithium toxicity at high doses
CNS disease	Use with caution - potential neurotoxicity

Resistance

Resistance involves:

Impaired oxygen-scavenging - higher local O2 concentrations prevent reductive activation
Lowered PFOR and ferredoxin levels - reduced drug activation
Decreased drug uptake

Clinically documented for T. vaginalis, G. lamblia, and anaerobic/microaerophilic bacteria.

Contraindications / Cautions

Pregnancy (first trimester): generally not advised
Hepatic impairment: reduce dose (drug is hepatically metabolized)
CNS disease: use cautiously due to neurotoxicity risk

Formulations

Oral: Tabs 250 mg, 500 mg; Caps 375 mg; Oral suspension 50 mg/mL
IV: 5 mg/mL (ready-to-use)
Topical gel: 0.75%, 1% (for rosacea)
Vaginal gel: 0.75%, 1.3%
Topical cream/lotion: 0.75%, 1%

Brand names: Flagyl, MetroGel, MetroLotion, MetroCream, Rosadan, Noritate, Vandazole, Nuvessa

Related 5-Nitroimidazoles

Tinidazole, secnidazole, ornidazole - closely related in structure and activity. Tinidazole (longer t1/2) is available in the U.S. and may be preferred for trichomoniasis and giardiasis due to equivalent or better efficacy with simpler dosing.

Recent evidence note: A 2025 systematic review and meta-analysis on the clinical pharmacokinetics of metronidazole (PMID 40741956) has been published, which may update population PK parameters. A separate 2024 meta-analysis on H. pylori resistance in China (PMID 39245326) highlights rising metronidazole resistance rates, which may affect eradication regimen selection.

Sources: Goodman & Gilman's The Pharmacological Basis of Therapeutics | Harriet Lane Handbook 23rd ed. | Medical Microbiology 9e | Sleisenger and Fordtran's GI and Liver Disease

Prescription for loose motion

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Excellent - I now have all I need for a comprehensive, textbook-grounded prescription guide. Here it is:

Prescription for Acute Diarrhea (Loose Motion) in Adults

Key principle: Most acute diarrhea is viral or self-limited bacterial - it resolves without antibiotics. Treatment is primarily supportive. Antibiotics are reserved for specific indications.

Step 1 - Assess Severity First

Feature	Action
Watery, no blood, no fever	Symptomatic treatment only
>6 stools/24 hr, signs of dehydration	Add oral/IV rehydration aggressively
Fever + bloody/mucoid stool (dysentery)	Consider empiric antibiotics; avoid loperamide
Severely ill, immunocompromised, elderly	Lower threshold for IV fluids + antibiotics

Prescription (Standard Adult Acute Diarrhea)

1. Oral Rehydration - FIRST AND MOST IMPORTANT

ORS (Oral Rehydration Solution)

Commercial sachets (e.g., Electral, WHO-ORS) dissolved in 1 L clean water
OR home formula: ½ tsp salt + 6 tsp sugar in 1 L clean water
Drink 200-400 mL after each loose stool
Continue until diarrhea resolves

2. Symptomatic - Antimotility Agent

(Only for mild-to-moderate watery diarrhea WITHOUT fever, blood, or mucus in stool)

Tab. Loperamide (Imodium) 2 mg

Initial dose: 4 mg (2 tabs) stat
Then: 2 mg after each unformed stool
Max: 16 mg/day
Duration: Until diarrhea stops, typically 1-2 days

AVOID loperamide in: fever, bloody stool, suspected Shiga toxin-producing E. coli (STEC), children <12 years

Alternative: Bismuth subsalicylate 524 mg every 30-60 min as needed, up to 8 doses/day (reduces symptoms, mild antisecretory effect)

3. Antispasmodic (for cramps)

Tab. Dicyclomine (Cyclopam) 20 mg or Tab. Mebeverine 135 mg

1 tablet TID before meals
For abdominal cramps/spasms

4. Empiric Antibiotic (if indicated - see below)

First-line: Tab. Azithromycin 500 mg OD x 3 days (or 1 g single dose for traveler's diarrhea)

Alternative: Tab. Ciprofloxacin 500 mg BD x 3-5 days (or Levofloxacin 500 mg OD x 3 days)

When to prescribe antibiotics:

Fever with bloody/mucoid stool (dysentery picture)
6 stools/24 hr with systemic symptoms
Immunocompromised patient
Traveler's diarrhea with fever ≥38.5°C
Suspected Shigella, Campylobacter, Salmonella (non-typhi with systemic features)

Per IDSA 2017 guidelines, empiric antibiotics do not dramatically alter the course of most diarrheal illness since the majority is viral or self-limited bacterial.

5. For Suspected Amoebic/Protozoal Cause

(mucoid stool, no fever initially, history of unhygienic food/water)

Tab. Metronidazole (Flagyl) 400 mg TID x 5-7 days

Follow with a luminal amebicide (Tab. Diloxanide furoate 500 mg TID x 10 days) to eliminate cysts

6. Probiotics (adjunct only)

Saccharomyces boulardii or Lactobacillus-based probiotic

May reduce duration by ~1 day
Note: Evidence does not support probiotics as primary treatment

Sample Prescription Card (Uncomplicated Watery Diarrhea, Adult)

Rx

1. ORS sachets - dissolve 1 sachet in 1L water; 
   drink 200 mL after each loose stool

2. Tab. Loperamide 2 mg
   - 2 tabs STAT, then 1 tab after each loose stool
   - Max 8 tabs/day x 2 days

3. Tab. Dicyclomine 20 mg
   - 1 tab TID x 3 days (for cramps)

4. Tab. Pantoprazole 40 mg (if nausea prominent)
   - 1 tab OD before breakfast

Advice:
- Light diet (rice, banana, toast, curd)
- Avoid dairy, spicy, fatty food
- Plenty of fluids
- Return if: blood in stool, high fever, 
  no improvement in 48 hr, signs of dehydration

Red Flags - Refer/Admit Immediately

Bloody diarrhea with high fever (dysentery)
Signs of severe dehydration (dry mouth, sunken eyes, no urine output, postural hypotension)
Immunocompromised or elderly with rapid deterioration
Suspected cholera (rice-water stools, profuse volume loss)
Diarrhea persisting >14 days (investigate for chronic cause)

Sources: Goldman-Cecil Medicine | Tintinalli's Emergency Medicine | Harrison's Principles of Internal Medicine 22E (2025) | Yamada's Textbook of Gastroenterology 7e - IDSA 2017 guidelines referenced throughout

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