Antimicrobials · Metronidazole and Tinidazole
The facts most likely to be tested
Press 1–5 to rate · ↑↓ to navigate
Metronidazole functions by forming toxic free radical metabolites that cause DNA strand breakage in susceptible organisms.
Nitroimidazoles are the first-line treatment for anaerobic infections below the diaphragm and protozoal infections including *Giardia*, *Entamoeba histolytica*, and *Trichomonas vaginalis*.
Patients must be strictly advised to avoid alcohol consumption during therapy and for 48 hours after completion to prevent a disulfiram-like reaction.
The most common gastrointestinal side effect is a metallic taste in the mouth, while the most serious neurological side effect is peripheral neuropathy or seizures with prolonged use.
Metronidazole is the preferred treatment for bacterial vaginosis and trichomoniasis, requiring the treatment of sexual partners to prevent reinfection.
Nitroimidazoles are a critical component of quadruple therapy for the eradication of *Helicobacter pylori* in patients with penicillin allergies.
Metronidazole is the drug of choice for the initial episode of non-severe Clostridioides difficile infection when oral vancomycin or fidaxomicin are unavailable or contraindicated.
Vignette unlocked
A 28-year-old female presents to the clinic complaining of malodorous, thin, gray vaginal discharge. On physical exam, the vaginal pH is 5.0, and a whiff test with potassium hydroxide is positive. Microscopic examination of the discharge reveals clue cells. She reports no known drug allergies but mentions she occasionally drinks wine with dinner.
What is the most appropriate pharmacologic management for this patient?
Oral metronidazole
The patient has bacterial vaginosis, which is treated with metronidazole; the clinician must counsel the patient on the disulfiram-like reaction associated with concurrent alcohol consumption.
Full handout
High yield triage
Classification
Nitroimidazole class; Metronidazole is the prototype.
Indications
Anaerobic infections, Trichomoniasis, Giardiasis, and C. difficile.
Mechanism of Action
Reductive activation by anaerobic organisms leads to DNA strand breakage.
Side Effects
Metallic taste, dark urine; Disulfiram-like reaction with alcohol.
Contraindications / Monitoring
First trimester pregnancy; monitor for peripheral neuropathy.
Full handout
Mechanism of Action
The drug acts as a prodrug, requiring reduction of the nitro group by ferredoxin or flavodoxin found only in anaerobes and protozoa. This reduction creates reactive nitro radical anions that bind to bacterial DNA. This process results in the inhibition of nucleic acid synthesis and subsequent cell death.
Unique Properties
These agents exhibit cidal activity specifically against obligate anaerobes and select protozoa. Unlike many antibiotics, they possess excellent tissue penetration, including the blood-brain barrier.
Indications
Primary treatment for Trichomoniasis (treat partners), Giardiasis, and Amebiasis. It is a key component in H. pylori eradication regimens and the treatment of Bacterial Vaginosis. It is also used for intra-abdominal anaerobic infections and as a second-line agent for C. difficile.
Pharmacokinetics
Metronidazole is well-absorbed orally and undergoes extensive hepatic metabolism. It is excreted primarily in the urine, which may cause a harmless red-brown discoloration. Dosage adjustments are rarely required for renal impairment but may be necessary in severe hepatic insufficiency.
Side Effects & Adverse Events
Common effects include nausea, headache, and a persistent metallic taste. Patients must be warned of the Disulfiram-like reaction (flushing, tachycardia, vomiting) if alcohol is consumed. Long-term use carries a risk of peripheral neuropathy and seizures.
Contraindications
First trimester pregnancy is a strict contraindication due to potential teratogenicity. Hypersensitivity to the drug class is an absolute barrier. Avoid use in patients with active CNS disease due to the risk of neurotoxicity.
Monitoring
Monitor for signs of peripheral neuropathy (numbness/tingling) during prolonged courses. Assess liver function tests in patients with known hepatic impairment. No routine blood count monitoring is required unless therapy is extended.
Clinical Pearls
Always remember the Disulfiram-like reaction—patients must avoid alcohol for 48 hours after the last dose. On boards, Metronidazole is the classic answer for foul-smelling vaginal discharge (BV or Trich) and foul-smelling diarrhea (Giardia).