Nervous System Effects: Serious adverse reactions include convulsive seizures and peripheral neuropathy. The latter is characterized mainly by numbness or paresthesia of an extremity. Dizziness, vertigo, incoordination, ataxia, confusion, irritability, depression, weakness, headache, tinnitus, hearing loss, and insomnia have also been reported with the use of metronidazole. Other effects include nervousness, malaise or syncope.
Gastrointestinal Effects: Nausea, accompanied by headache, anorexia, dry mouth and sharp, unpleasant metallic taste, vomiting, abdominal discomfort, and diarrhea have been reported with the use of metronidazole. Occasionally, epigastric distress and constipation; rarely, pseudomembranous colitis has also been reported.
Rare cases of acute pancreatitis, which generally abated on withdrawal of the drug, have been reported.
Hematologic Effects: Reversible neutropenia (leukopenia); rarely, reversible thrombocytopenia. Very rare cases of agranulocytosis were also reported. One patient experienced bone marrow aplasia with metronidazole therapy.
Cardiovascular Effects: Flattening of the T-wave may be seen in electrocardiographic tracings.
Sensitivity Reactions: Urticaria, pruritus, maculopapular rash, erythematous rash, flushing, nasal congestion, dryness of mouth (or vagina or vulva), fever, and fleeting joint pains sometimes resembling "serum sickness". Aseptic meningitis and photosensitivity reactions have occurred rarely.
Genitourinary Effects: Dysuria, cystitis, polyuria, incontinence, and a sense of pelvic pressure. Urethral burning or discomfort, dryness of the vagina, proliferation of Candida in the vagina, dyspareunia, decreased libido, proctitis, have also occurred. Genital pruritus, dysmenorrhea and urinary tract infection have also been reported.
Instances of darkened urine have also been reported. Although the pigment which is probably responsible for this phenomenon has not been identified, it is most certainly a metabolite of metronidazole and seems to have no clinical significance.
Respiratory Effects: Upper respiratory tract infections, rhinitis, sinusitis, and pharyngitis.
Others: Thrombophlebitis has been reported after IV infusion of metronidazole. Patients taking metronidazole and alcoholic beverages concomitantly may experience abdominal distress, nausea, vomiting, flushing, or headache. A modification of the taste of alcoholic beverages has also been reported.