From the Guidelines
Metronidazole is associated with several side effects, including gastrointestinal issues, neurological problems, and interactions with alcohol, which patients should be aware of before starting treatment. The most frequent side effects of metronidazole include:
- A metallic taste in the mouth
- Nausea
- Vomiting
- Diarrhea
- Abdominal discomfort Patients may also experience:
- Headaches
- Dizziness
- Dry mouth A particularly important warning is that metronidazole interacts with alcohol, causing a disulfiram-like reaction with symptoms such as flushing, headache, nausea, vomiting, and abdominal cramps 1. Therefore, patients should avoid alcohol during treatment and for at least 48 hours after completing the medication. Less common but serious side effects include:
- Peripheral neuropathy (numbness or tingling in hands or feet)
- Seizures
- Encephalopathy Metronidazole can also cause a darkening of urine, which is harmless but may alarm patients if they're not forewarned 1. Some patients develop a rash or itching, indicating an allergic reaction that requires medical attention. Long-term use increases the risk of certain side effects, particularly neurological problems. According to a recent study, metronidazole should be limited to the treatment of an initial episode of mild-moderate CDI, and repeated or prolonged courses of metronidazole should be avoided due to the risk of cumulative and potentially irreversible neurotoxicity 1.
From the FDA Drug Label
ADVERSE REACTIONS Two serious adverse reactions reported in patients treated with metronidazole have been convulsive seizures and peripheral neuropathy, the latter characterized mainly by numbness or paresthesia of an extremity Since persistent peripheral neuropathy has been reported in some patients receiving prolonged administration of metronidazole, patients should be specifically warned about these reactions and should be told to stop the drug and report immediately to their physicians if any neurologic symptoms occur The most common adverse reactions reported have been referable to the gastrointestinal tract, particularly nausea reported by about 12% of patients, sometimes accompanied by headache, anorexia, and occasionally vomiting; diarrhea; epigastric distress; and abdominal cramping. Constipation has also been reported The following reactions have also been reported during treatment with metronidazole: Mouth: A sharp, unpleasant metallic taste is not unusual. Furry tongue, glossitis, and stomatitis have occurred; these may be associated with a sudden overgrowth of Candida which may occur during therapy. Hematopoietic:Reversible neutropenia (leukopenia); rarely, reversible thrombocytopenia Cardiovascular:Flattening of the T‘wave may be seen in electrocardiographic tracings. Central Nervous System:Convulsive seizures, peripheral neuropathy, dizziness, vertigo, incoordination, ataxia, confusion, irritability, depression, weakness, and insomnia. Hypersensitivity:Urticaria, erythematous rash, flushing, nasal congestion, dryness of the mouth (or vagina or vulva), and fever Renal:Dysuria, cystitis, polyuria, incontinence, and a sense of pelvic pressure. Instances of darkened urine have been reported by approximately one patient in 100,000. Although the pigment which is probably responsible for this phenomenon has not been positively identified, it is almost certainly a metabolite of metronidazole and seems to have no clinical significance Other:Proliferation of Candida in the vagina, dyspareunia, decrease of libido, proctitis, and fleeting joint pains sometimes resembling "serum sickness. " If patients receiving metronidazole drink alcoholic beverages, they may experience abdominal distress, nausea, vomiting, flushing, or headache. A modification of the taste of alcoholic beverages has also been reported. Rare cases of pancreatitis, which generally abated on withdrawal of the drug, have been reported Crohn's disease patients are known to have an increased incidence of gastrointestinal and certain extraintestinal cancers. There have been some reports in the medical literature of breast and colon cancer in Crohn's disease patients who have been treated with metronidazole at high doses for extended periods of time. A cause and effect relationship has not been established. Crohn's disease is not an approved indication for metronidazole.
The side effects of metronidazole include:
- Gastrointestinal: nausea, diarrhea, epigastric distress, abdominal cramping, constipation
- Central Nervous System: convulsive seizures, peripheral neuropathy, dizziness, vertigo, incoordination, ataxia, confusion, irritability, depression, weakness, and insomnia
- Hematopoietic: reversible neutropenia, reversible thrombocytopenia
- Hypersensitivity: urticaria, erythematous rash, flushing, nasal congestion, dryness of the mouth, fever
- Renal: dysuria, cystitis, polyuria, incontinence, pelvic pressure
- Other: proliferation of Candida, dyspareunia, decrease of libido, proctitis, joint pains, pancreatitis 2
From the Research
Side Effects of Metronidazole
The side effects of metronidazole can be categorized into mild to moderate and serious effects.
- Mild to moderate side effects include:
- Serious side effects include:
Special Considerations
Metronidazole has been associated with mutagenicity and carcinogenicity in some animal models, although the risk in humans is considered low 4, 6.