The most common adverse effects of metronidazole are gastrointestinal disturbances mostly nausea and unpleasant metallic taste. Diarrhea, constipation and vomiting may also be experienced. A furred or coated tongue, glossitis and stomatitis may be associated with overgrowth of Candida sp. There were rare reports of antibiotic-associated colitis. The adverse effects are generally dose-related.
Headache, weakness, dizziness, drowsiness, ataxia, dysarthria, insomnia, changes in mood or mental state such as depression and confusion may occur, seizure and peripheral neuropathy may occur after prolonged therapy with high doses of metronidazole.
Temporary moderate leucopenia, thrombocytopenia, skin rashes, urticaria, pruritus may occur occasionally, gynecomastia, erythema multiforme, angioedema and anaphylaxis have been reported rarely.
Other adverse effects include optic neuropathies, urethral discomfort, darkening of urine, elevated liver enzymes, cholestatic hepatitis and jaundice.