4. Toxicity—Adverse effects include gastrointestinal irritation, headache, and dark coloration of urine. More serious toxicity includes leukopenia, dizziness, and ataxia. Opportunistic fungal infections may occur during treatment with metronidazole and tinidazole. Drug interactions with metronidazole include a disulfiram-like reaction with ethanol and potentiation of coumarin anticoagulant effects. Although metronidazole and tinidazole are not contraindicated in pregnancy, the drugs should be used with caution.