Common Side Effects of Metronidazole
The most common side effects of metronidazole are gastrointestinal disturbances, particularly nausea (occurring in approximately 12% of patients), along with metallic taste, headache, and abdominal discomfort. 1
Gastrointestinal Side Effects (Most Frequent)
The gastrointestinal tract is the primary system affected by metronidazole:
- Nausea occurs in about 12% of patients and represents the most frequently reported adverse effect 1
- Metallic taste is not unusual and commonly accompanies therapy 1
- Abdominal cramping and epigastric distress are frequently reported 1
- Diarrhea occurs commonly, though metronidazole is also used to treat certain diarrheal illnesses 1
- Anorexia and vomiting may accompany nausea 1
- Constipation has also been documented 1
The CDC guidelines specifically note that metronidazole has a "high frequency of gastrointestinal side effects" compared to alternative agents like tinidazole 2. Some providers prefer intravaginal routes when available specifically to avoid these systemic gastrointestinal effects 2.
Oral and Taste-Related Effects
- Sharp, unpleasant metallic taste is a characteristic side effect 1, 3
- Furry tongue, glossitis, and stomatitis may occur, sometimes associated with Candida overgrowth during therapy 1
- Dry mouth has been reported 1
- Modification of the taste of alcoholic beverages occurs 1
Neurological Side Effects (Less Common but Important)
While less frequent than gastrointestinal effects, neurological symptoms warrant attention:
- Headache is commonly reported alongside nausea 1, 3
- Dizziness and vertigo can occur 1, 4
- Peripheral neuropathy characterized by numbness or paresthesia of extremities is a serious concern, particularly with prolonged use 1, 5, 6
- Ataxia and incoordination may develop 1, 4
The FDA label specifically warns that patients should be told to stop the drug immediately and report to their physicians if any neurologic symptoms occur 1.
Genitourinary Effects
- Darkened urine occurs in approximately one patient in 100,000 1, 7
- Dysuria, cystitis, and polyuria have been reported 1
- Sense of pelvic pressure may occur 1
Alcohol Interaction
- Disulfiram-like reaction when combined with alcohol, causing abdominal distress, nausea, vomiting, flushing, or headache 1, 3
Important Clinical Considerations
Patients should avoid alcohol during metronidazole treatment and for 24 hours after completion to prevent the disulfiram-like reaction 2, 1. The gastrointestinal side effects are generally mild to moderate and dose-related 3. While serious neurotoxicity is rare, it can occur and is typically reversible upon discontinuation 5, 4. Prolonged or repeated courses should be avoided due to cumulative neurotoxicity risk 2.