Review allergy history, pregnancy status, and alcohol use before starting metronidazole.
Absolute: prior immediate hypersensitivity to metronidazole or other nitroimidazoles. Relative: first-trimester pregnancy, severe hepatic disease, chronic alcohol use, history of blood dyscrasias, and concomitant QT-prolonging agents.
Hypersensitivity to metronidazole or other nitroimidazoles — prior anaphylaxis, severe cutaneous eruptions, or neuropathic reactions require complete avoidance and allergy flagging.
First-trimester pregnancy unless treatment is urgent; reserve for serious infections under obstetric guidance because placental exposure during organogenesis has limited safety data.
Severe hepatic impairment, which prolongs half-life and raises neurotoxicity risk; employ lower doses with therapeutic drug monitoring when available.
History of blood dyscrasias, including leukopenia; obtain baseline counts and repeat testing if therapy exceeds 10 days.
Concomitant alcohol or disulfiram use, as metronidazole can precipitate disulfiram-like reactions and acute psychosis — enforce a 48-hour alcohol-free window before and at least 72 hours after therapy.