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|Systematic (IUPAC) name|
|Trade names||Flagyl, Filmet|
|Pregnancy cat.||B (US) B2 (Au)|
|Legal status||Prescription Only (S4) (AU) POM (UK) ℞-only (US)|
|Routes||oral, topical, rectal, IV, vaginal|
|Excretion||Renal (60-80%), biliary (6–15%)|
|ATC code||A01 , D06, G01, J01, P01, QP51|
|Mol. mass||171.15 g/mol|
|Melt. point||159–163 °C (318–325 °F)|
| (what is this?)
Metronidazole (INN) (pron.: //) (Flagyl, and others) is a nitroimidazole antibiotic medication used particularly for anaerobic bacteria and protozoa. Metronidazole is an antibiotic, amebicide, and antiprotozoal. It is the drug of choice for first episodes of mild-to-moderate Clostridium difficile infection. It is marketed in the U.S.A. by Pfizer and globally by Sanofi under the trade name Flagyl, and is also sold under other brand names. Metronidazole was developed in 1960.
Metronidazole is used also as a gel preparation in the treatment of the dermatological conditions such as rosacea (Rozex and MetroGel by Galderma) and fungating tumours (Anabact, Cambridge Healthcare Supplies).
Medical uses 
Metronidazole is indicated for the treatment of:
- Bacterial vaginosis, commonly associated with overgrowth of Gardnerella species and coinfective anaerobes (Mobiluncus, Bacteroides), in symptomatic patients
- Pelvic inflammatory disease in conjunction with other antibiotics such as ofloxacin, levofloxacin, or ceftriaxone
- Anaerobic infections such as Bacteroides fragilis, spp, Fusobacterium spp, Clostridium spp, Peptostreptococcus spp, Prevotella spp, or any other anaerobes in intra-abdominal abscess, peritonitis, diverticulitis, empyema, pneumonia, aspiration pneumonia, lung abscess, diabetic foot ulcer, meningitis and brain abscesses, bone and joint infections, septicemia, endometritis, or endocarditis
- Dental infection of bacterial origin, such as periapical abscess, periodontal abscess, acute pericoronitis of impacted or partially erupted teeth; often used in conjunction with Amoxicillin
- Giardiasis: infection of the small intestine caused by the ingestion of infective cysts of protozoan Giardia lamblia.
- Trichomoniasis: infection caused by Trichomonas vaginalis, which is a common cause of vaginitis and is the most frequently presenting new infection of the common sexually transmitted diseases.
- Prophylaxis for those undergoing potentially contaminated colorectal surgery or appendectomies and may be combined with neomycin
- Crohn's disease with colonic or perianal involvement (non-FDA approved) – believed to be more effective in combination with ciprofloxacin
- Topical metronidazole is indicated for the treatment of rosacea, and in the treatment of malodorous fungating wounds.
Preterm births 
Metronidazole has also been used in women to prevent preterm birth associated with bacterial vaginosis, amongst other risk factors including the presence of cervicovaginal fetal fibronectin (fFN). A randomised controlled trial demonstrated that metronidazole was ineffective in preventing preterm delivery in high-risk pregnant women and, conversely, the incidence of preterm delivery was actually higher in women treated with metronidazole.
In a study it has been found that metronidazole is not the right antibiotic to administer in these circumstances and that it was often administered too late to be of use. Clindamycin administered early in the second trimester to women who test positive for bacterial vaginosis seemed to be more effective.
Veterinary use 
Metronidazole is not labeled for animal use but is widely used to treat infections of Giardia in dogs, cats, and other companion animals, although it does not reliably clear infection with this organism and is being supplanted by Fenbendazole for this purpose in dogs and cats. Metronidazole is also used for the management of chronic inflammatory bowel disease in cats and dogs.  Another common usage is the treatment of systemic and/or GI clostridial infections in horses. Metronidazole or simply "Metro" is used in the aquarium hobby to treat ornamental fish and as a wide spectrum treatment for bacterial and protozoan infections in reptiles and amphibians. It is also used to treat human enteric (gi) and systemic infections. In general, the veterinary community may use metronidazole for any potentially susceptible anaerobic infection. The U.S. Food and Drug Administration (FDA) prohibits the use of metronidazole in food animals.
Adverse effects 
Common adverse drug reactions (≥1% of patients) associated with systemic metronidazole therapy include: nausea, diarrhea, and/or metallic taste in the mouth. Intravenous administration is commonly associated with thrombophlebitis. Infrequent adverse effects include: hypersensitivity reactions (rash, itch, flushing, fever), headache, dizziness, vomiting, glossitis, stomatitis, dark urine, and/or paraesthesia.
Metronidazole is listed by the US National Toxicology Program (NTP) as reasonably anticipated to be a human carcinogen. Although some of the testing methods have been questioned, oral exposure has been shown to cause cancer in experimental animals. The relationship between exposure to metronidazole and human cancer is unclear. One study (Beard et al. 1988) found an excess in lung cancer among women (even after adjusting for smoking), while other studies (IARC 1987; Thapa et al. 1998) found either no increased risk, or a statistically insignificant risk.  Metronidazole is listed as a possible carcinogen according to the WHO International Agency for Research on Cancer (IARC).
Due to its potential carcinogenic properties, metronidazole is banned in the EU and the USA for veterinary use in the feed of animals and is banned for use in any food animals in the USA. In the USA, this type of restriction is covered under the Delaney clause.
Earlier studies suggested a relation between metronidazole and various birth defects. Those studies are now considered flawed and more recent studies "do not support a significant increased risk for birth defects or other adverse effects on the fetus."
Common adverse drug reactions associated with topical metronidazole therapy include local redness, dryness, and/or skin irritation; and eye watering (if applied near eyes).
Central nervous system 
Toxic levels of metronidazole can cause symmetrical lesions in the brain in the corpus callosum and dentate nuclei. Metronidazole toxicity is rare (though the actual incidence is not known with certainty). Patients present with nausea, vomiting, dysarthria, vertigo, and confusion. Other side effects of the metronidazole can include dry mouth, diarrhea, headache, dizziness, or peripheral neuropathy. An examination of a patient reveals that the patient is confused and has dysarthria (difficult or unclear articulation of speech that is otherwise linguistically normal), ataxia (loss of full control of bodily movements), abnormal eye movements including nystagmus and ophthalmoparesis. Magnetic resonance imaging (MRI) most often shows bilateral symmetric fluid-attenuated inversion recovery (FLAIR) hyperintense lesions of the dentate nuclei (which is one of the deep cerebellar nuclei), as well as symmetric lesions of the corpus callosum and basal ganglia. The brain lesions seen on the MRI rarely enhance and may be Diffusion-Weighted Imaging (DWI) hyperintense. It has a subacute to acute course. Most reports have been seen in patients who receive approximately one gram a day of metronidazole for over 30 days.
Metronidazole can rarely cause central nervous system toxicity; it does not seem to be a dose- or duration-related phenomenon. Most patients will have MRI abnormalities. Prognosis is excellent with metronidazole cessation.
Interaction with alcohol 
Consuming ethanol (alcohol) while taking metronidazole has long been thought to have a disulfiram-like reaction with effects that can include nausea, vomiting, flushing of the skin, tachycardia (accelerated heart rate), and shortness of breath. Typically a 2006 Medicines Handbook warns that consumption of alcohol should be avoided by patients during systemic metronidazole therapy and for at least 48 hours after completion of treatment. However there are studies calling into question the interaction of alcohol and metronidazole, and the mechanism of this reaction in the clinical setting has been questioned by some authors, and a possible central toxic serotonin reaction for the alcohol intolerance suggested. Metronidazole is also generally thought to inhibit the liver metabolism of propylene glycol (found in some foods, medicines and in many electronic cigarette e-liquids), and thus propylene glycol may potentially have similar interaction effects with metronidazole.
Stevens–Johnson syndrome 
Serotonin syndrome 
Serotonin syndrome has been reported with metronidazole. Symptoms may appear within minutes of oral dosage, and/or days to months. Signs and symptoms may include: increased heart rate, shivering, sweating, dilated pupils, twitching, over-responsive reflexes, hyperthermia, fever, confusion, hyper-vigilance, agitation, muscle rigidity, headache, elevated blood pressure, and changes in blood chemistry. The only direct treatment is to discontinue the offending drugs.
Mechanism of action 
Metronidazole, taken up by diffusion, is selectively absorbed by anaerobic bacteria and sensitive protozoa. Once taken up by anaerobes, it is non-enzymatically reduced by reacting with reduced ferredoxin, which is generated by pyruvate oxido-reductase. Many of the reduced nitroso intermediates will form sulfinamides and thioether linkages with cysteine-bearing enzymes, thereby deactivating these critical enzymes. As many as 150 separate enzymes are affected.
In addition or alternatively, the metronidazole metabolites are taken up into bacterial DNA, and form unstable molecules. This function only occurs when metronidazole is partially reduced, and because this reduction usually happens only in anaerobic cells, it has relatively little effect upon human cells or aerobic bacteria.
2-Methylimidazole (1) may be prepared via the Debus-Radziszewski imidazole synthesis, or from ethylenediamine and acetic acid, followed by treatment with lime, then Raney nickel. 2-Methylimidazole nitrated to give 2-methyl-4(5)-nitroimidazole (2), which is in turn alkylated with ethylene oxide or 2-chloroethanol to give metronidazole (3):
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