Side Effects of Metronidazole
Metronidazole commonly causes gastrointestinal side effects including nausea, vomiting, and abdominal discomfort, while serious adverse effects include peripheral neuropathy and disulfiram-like reactions with alcohol. 1
Common Side Effects
Gastrointestinal effects: The most frequently reported adverse reactions (approximately 12% of patients) include nausea, sometimes accompanied by headache, anorexia, vomiting, diarrhea, epigastric distress, and abdominal cramping 1
Taste disturbances: A sharp, unpleasant metallic taste is common during treatment 1
Oral manifestations: Furry tongue, glossitis, and stomatitis may occur due to overgrowth of Candida during therapy 1
Vaginal candidiasis: Proliferation of Candida in the vagina can occur during treatment 1, 2
Neurological Side Effects
Peripheral neuropathy: This serious adverse effect is characterized mainly by numbness or paresthesia of extremities, particularly with prolonged administration 1, 2
Central nervous system effects: Dizziness, vertigo, incoordination, ataxia, confusion, irritability, depression, weakness, and insomnia may occur 1
Convulsive seizures: Rarely reported but serious neurological complication 1
Risk increases with duration: Neurotoxicity risk increases with prolonged courses, and repeated or prolonged courses should be avoided 2
Disulfiram-like Reaction
Alcohol interaction: Patients who consume alcoholic beverages while taking metronidazole may experience abdominal distress, nausea, vomiting, flushing, or headache 1
Taste alteration: Modification of the taste of alcoholic beverages has been reported 1
Other Significant Side Effects
Hematologic effects: Reversible neutropenia (leukopenia) and rarely, reversible thrombocytopenia 1
Cardiovascular effects: Flattening of the T-wave may be seen in electrocardiographic tracings 1
Hypersensitivity reactions: Urticaria, erythematous rash, flushing, nasal congestion, dryness of the mouth (or vagina or vulva), and fever 1
Genitourinary effects: Dysuria, cystitis, polyuria, incontinence, and a sense of pelvic pressure 1
Urine discoloration: Darkened urine has been reported in approximately 1 in 100,000 patients 1, 3
Special Considerations
Pregnancy
- Metronidazole is contraindicated during the first trimester of pregnancy 2
- For pregnant women with trichomoniasis who are symptomatic, treatment with 2g of metronidazole in a single dose may be considered after the first trimester 2
Prolonged Use
- Prolonged or repeated courses of metronidazole should be avoided due to risk of cumulative and potentially irreversible neurotoxicity 2
- High doses (6-10.4g) given over 5-7 days have been associated with neurotoxic effects including seizures and peripheral neuropathy 1
Treatment Failures
- For trichomoniasis treatment failures, higher doses or longer durations of metronidazole may be required 2
- If initial treatment fails, patients should be retreated with metronidazole 500mg twice daily for 7 days 2
- For persistent failures, a single 2g dose of metronidazole once daily for 3-5 days may be needed 2
Monitoring and Management
Patients should be specifically warned about neurologic symptoms and instructed to stop the drug and report immediately to their physicians if any neurologic symptoms occur 1
For overdose, there is no specific antidote; management consists of symptomatic and supportive therapy 1
Patients with an immediate-type allergy to metronidazole may require desensitization 2
For patients with diminished susceptibility to metronidazole, higher doses may be effective, but consultation with specialists is recommended for persistent treatment failures 2