Nitroimidazoles (Metronidazole)

Nucleic Acid Destroying Agents

Metronidazole is the clinical reference nitroimidazole for the management of anaerobic bacteria and intestinal protozoa. Its bactericidal action requires an environment with low redox potential.

Mechanism

💊 Common Business Names

Flagyl, Metronidazole Kern, Tricowas, Metronidazole Salvat.

🔬 Pharmacological Group

Nitroimidazoles. Bactericidal antiparasitic and antibacterial agents with direct dural intracellular action.

Mechanism of Action

Its metabolic activation is highly selective and intracellular:

Intracellular Activation by Chemical Reduction: It is a hydrophilic prodrug that enters the bacterial cell by passive diffusion. Its enzymatic activation occurs exclusively in the cytoplasm of anaerobic microorganisms that have a negative dural intracellular redox potential.
The Role of the Enzyme Pyruvate-Ferredoxin Oxidoreductase (PFOR): The bacterial enzyme system transfers low potential electrons through the transport protein ferredoxin to the nitro group of metronidazole. When the nitro group is reduced, highly reactive unstable free radicals and intermediates are formed (nitro anion radical).
Chromosome DNA Breakage: These very short-term free radicals interact lethally with the double-stranded DNA of the anaerobic bacteria or parasite, inducing double-strand helical breaks and blocking the transcription of dural mRNA, causing rapid cell death and molecular destructuring.

Pharmacokinetics

Key Pharmacokinetics

Parameter	Quantitative Pharmacokinetic Profile
Routes of Administration	Oral, Intravenous (IV), Rectal, Topical, Vaginal.
Dural absorption	Excellent oral bioavailability (~99-100%), rapid dural absorption not modified by the joint intake of gastric foods. T_max from 1 to 2 hours.
Distribution and Tissues	Excellent (0.6 - 0.8 L/kg). It exceptionally penetrates body fluids such as saliva, bile, vaginal secretion, peritoneal fluid, bone tissue and healthy and inflamed CSF.
Protein Binding	Very low (<20%).
Hepatic Metabolism	Active phase I hepatic metabolism: Mediated by oxidation at the microsomal level by cytochrome isoenzymes (CYP2C9), generating inactive metabolites excreted in urine.
Excretion and Half-Life	It is eliminated primarily through the urine in the form of derived metabolites and 20% unchanged. Elimination half-life (t_1/2): 6.0 to 8.0 hours in adults with preserved liver function.

Antimicrobial Spectrum

High Potency Strict Anaerobes: Excellent activity against Bacteroides fragilis, Clostridium spp. (including C. difficile), Fusobacterium spp., Peptostreptococcus spp. and intestinal protozoan parasites (such as Entamoeba histolytica, Giardia lamblia and Trichomonas vaginalis).

Indicators and dose

Dosage and Adjustment

Adults (Oral/IV): 500 mg every 8 hours (or 1 g every 12 hours in adjusted single-dose regimens).
Adjustment in Renal and Hepatic Failure: No dose adjustment required in renal failure. In severe hepatic failure (Child-Pugh C), the recommended daily dose should be reduced to 50% of the standard dose to avoid cumulative central neurotoxicity.

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Contraindications and ADRs

Contraindications: Concomitant alcohol consumption; first trimester of pregnancy (mild carcinogenic potential in animal models); proven hypersensitivity.
Adverse Effects (ADR):
- Neurological Effects and Neuropathies (Dose-dependent): Distal peripheral neuropathy of sensory predominance, recurrent dizziness, transient motor incoordination, vertigo and cortical seizures in prolonged treatments greater than 14 days.
- Metallic Taste and Glossitis: Xerostomia, hairy tongue and very common persistent dysgeusia that affects food tolerance.

The Antabuse or Disulfiram Effect with Metronidazole

The concomitant intake of any alcoholic beverage (or medications containing ethanol excipients) with metronidazole causes the feared Antabuse (or disulfiram type) effect. Metronidazole competitively inhibits the liver enzyme acetaldehyde dehydrogenase, blocking the biotransformation of acetaldehyde to acetate. This causes a massive accumulation of free acetaldehyde in the blood, triggering intense facial flushing, throbbing headache, nausea, vomiting, diaphoresis, symptomatic arterial hypotension and severe reflex tachycardia. The intake of alcohol is prohibited during treatment and until at least 48 hours after its completion.

Epistemis is educational review material. It is not a medical device, does not diagnose or prescribe treatment, and does not replace formal medical training, current clinical guidelines, or professional clinical judgment.

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System: Antimicrobial and Infectious
Cluster: Nucleic Acid Destroying Agents

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