Adverse Effects
Common adverse drug reactions (≥1% of patients) associated with systemic metronidazole therapy include: nausea, diarrhea, and/or metallic taste in the mouth. Intravenous administration is commonly associated with thrombophlebitis. Infrequent adverse effects include: hypersensitivity reactions (rash, itch, flushing, fever), headache, dizziness, vomiting, glossitis, stomatitis, dark urine, and/or paraesthesia.
High doses and/or long-term systemic treatment with metronidazole is associated with the development of leukopenia, neutropenia, increased risk of peripheral neuropathy and/or CNS toxicity.
Metronidazole is listed by the US National Toxicology Program (NTP) as reasonably anticipated to be a human carcinogen. Although some of the testing methods have been questioned, oral exposure has been shown to cause cancer in experimental animals. The relationship between exposure to metronidazole and human cancer is unclear. One study (Beard et al. 1988) found an excess in lung cancer among women (even after adjusting for smoking), while other studies (IARC 1987; Thapa et al. 1998) found either no increased risk, or a statistically insignificant risk. Metronidazole is listed as a possible carcinogen according to the WHO International Agency for Research on Cancer (IARC).
Due to its potential carcinogenic properties, metronidazole is banned in the EU and the USA for veterinary use in the feed of animals and is banned for use in any food animals in the USA. In the USA, this type of restriction is covered under the Delaney clause.
Earlier studies suggested a relation between metronidazole and various birth defects. Those studies are now considered flawed and more recent studies "do not support a significant increased risk for birth defects or other adverse effects on the fetus."
Common adverse drug reactions associated with topical metronidazole therapy include local redness, dryness, and/or skin irritation; and eye watering (if applied near eyes).
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