Metronidazole is one of the mainstay drugs for the treatment of anaerobic bacterial infections, protozoal infections, and microaerophilic bacterial infections. It is cytotoxic to facultative anaerobic microorganisms.
Metronidazole is FDA-approved for the treatment of protozoal infections such as Trichomoniasis vaginalis, Entamoeba histolytica, Giardia lamblia, blastocysts, and Balantidium coli. It is also FDA approved to treat anaerobic bacterial infections caused by Bacteroides species, Fusobacterium species, Clostridium species, Gardnerella vaginalis, Helicobacter pylori, Prevotella species, Porphyromonas species, and Biophilia wadsworthia. Therefore, it is not surprising that metronidazole is widely accepted and FDA-approved for the treatment of a broad range of infections: intestinal amebiases, liver amebiasis, bacterial septicemia, bone and joint infections, central nervous system (CNS) infections (meningitis and brain abscess), endocarditis, gynecologic infections (endometritis, tubo-ovarian abscess, bacterial vaginosis), intra-abdominal infections, lower respiratory tract infections, skin structure infections, and surgical prophylaxis (colorectal surgeries).
Metronidazole has additional off-label uses in the management of other conditions and infections including balantidiases, bite wound infections, animal and human bites, clostridioides (formerly Clostridium difficile), Crohn disease, post-surgical resection management, perianal fistulas, Dietamoeba fragilis infections, giardiasis, Helicobacter pylori eradication, periodontitis, post ileal pouch-anal anastomosis (pouchitis), and tetanus.
Metronidazole has rapid bactericidal effects against anaerobic bacteria with a killing rate proportional to the drug concentration. Concentration-dependent bactericidal properties have been demonstrated against Entamoeba histolytica and Trichomonas vaginalis. Furthermore, it kills Bacteroides fragilis and Clostridium perfringens more rapidly than treatment doses of clindamycin. It also penetrates the blood-brain barrier.
Metronidazole diffuses into the organism, inhibits protein synthesis by interacting with DNA and causing a loss of helical DNA structure and strand breakage. Therefore, it causes cell death in susceptible organisms.
The mechanism of action of metronidazole occurs through a four-step process. Step one is the entry into the organism by diffusion across the cell membranes of anaerobic and aerobic pathogens. However, antimicrobial effects are limited to anaerobes. Step two involves reductive activation by intracellular transport proteins by altering the chemical structure of pyruvate-ferredoxin oxidoreductase. Reduction of metronidazole creates a concentration gradient in the cell that drives uptake of more drug and promotes free radical formation that is cytotoxic. Step three, interactions with intracellular targets, is achieved by cytotoxic particles interacting with host cell DNA resulting in DNA strand breakage and fatal destabilization of the DNA helix. Step four is the breakdown of cytotoxic products. Metronidazole is also cytotoxic to facultatively anaerobic bacteria like Helicobacter pylori and Gardnerella vaginalis, but the mechanism of action to these pathogens is not well understood.
Metronidazole may be administered orally, intravenously, or topically. It comes in capsule, tablet, topical and intravenous forms. Standard capsule dosing is 375 mg. Standard tablet dosing is 250 mg or 500 mg. Standard intravenous preparation and dosing is 5mg/mL (100mL) and 500 mg (100 mL), respectively. When administered intravenously, the drug solution should not come in contact with equipment containing aluminum. Infusion of the intravenous solution should be over 30 to 60 minutes. Oral administration can be taken with food to help minimize stomach discomfort. The extended-release tablets should be administered on an empty stomach 1 hour before or 2 hours after meals. It should not be split or crushed. Topical application gel is 0.75% and 37.5 mg per applicator and is applied vaginally.
The primary adverse effects of metronidazole include confusion, peripheral neuropathy, metallic taste, nausea, vomiting, and diarrhea. Adverse events seen in greater than 10% of the population include headache (18%), vaginitis (15%), and nausea (10% to 12%). Adverse events affecting less than 10% of the population are metallic taste (9%), dizziness (4%), genital pruritus (5%), abdominal pain (4%), diarrhea (4%), xerostomia (2%), dysmenorrhea (3%), urine abnormality (3%), urinary tract infection (2%), bacterial infection (7%), candidiasis (3%), flu-like symptoms (6%), upper respiratory tract infection (4%), pharyngitis (3%), and sinusitis (3%). Rarely, there are reports of transient leukopenia and neutropenia as well.
Metronidazole comes with a black box warning that it may be carcinogenic based on some animal studies in mice and rats. However the risks are considered low, and additional follow-up studies of patients treated do not reveal an increased incidence of cancer. As with any medication choice, physicians and patients must decide whether the benefit from therapy outweighs the potential risk. Use of metronidazole should be reserved for conditions approved by the FDA; it should not be used prophylactically or unnecessarily.
Additional warnings and precautions for metronidazole exist. Prolonged courses of the drug can cause severe neurological disturbances due to the risk of cumulative neurotoxicity. Monitor for neurologic sequela and discontinue therapy if any abnormal neurologic symptoms occur. Prolonged use may also result in fungal or bacterial superinfection, including C. difficile-associated diarrhea (CDAD) and pseudomembranous colitis. There are reports of CDAD even after more than 2 months of postantibiotic treatment. Candidiasis infection may also be more prominent during metronidazole treatment.
Metronidazole is contraindicated in patients with documented hypersensitivity to the drug or its components, and it should be avoided in first-trimester pregnancy. Patients should also avoid consuming alcohol or products containing propylene glycol while taking metronidazole and within three days of therapy completion. Metronidazole is likewise contraindicated if there has been recent disulfiram use within the past two weeks.
During and after prolonged therapy or repeated courses, complete blood count (CBC) with differential requires monitoring. Carefully observe patients for the onset of neurologic symptoms and consider discontinuation of metronidazole when or if new neurologic symptoms occur. Elderly patients, as well as previously diagnosed patients with severe hepatic impairment and/or end-stage renal disease, should also be monitored closely.
There have been reports of disulfiram-like reaction in patients drinking ethanol while administered systemic or vaginal metronidazole. A typical disulfiram reaction causes flushing, tachycardia, palpitations, nausea, and vomiting. Alcohol should be avoided during treatment and from up to forty-eight hours to fourteen days after treatment completion depending on the source; the manufacturer product information recommends avoiding alcohol ingestion during metronidazole therapy and for at least 48 hours afterward. Ethanol-containing medications such as elixirs as well as tipranavir, capsules, intravenous (IV) anidulafungin, IV trimethoprim-sulfamethoxazole, and many cough/cold syrups can also lead to a disulfiram-like reaction when ingested with metronidazole.
There should be judicious prescribing of metronidazole by providers only for known indications and high clinical suspicion for needing treatment of anaerobic bacterial infections, protozoal infections, and microaerophilic bacterial infections. Overprescribing may contribute to increasing future antibiotic resistance to the drug. Documentation already exists regarding increased resistance to metronidazole in the treatment of C. difficile infections. Also, prescribers should routinely provide education to patients to abstain from alcoholic beverages while taking metronidazole, which in turn will help lead to fewer disulfiram reaction symptoms. The team of health professionals including physicians, nurses, and pharmacists must work together to provide the best care for these patients when using this drug. [Level V]
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