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More information on metronidazole (Flagyl)

Letter in the Australian Prescriber: Drugs and Breastfeeding

May 9, 2005

The following letter by Elizabeth Barbehenn, Ph.D., a Public Citizen research analyst, appeared in Australian Prescriber.

Editor — After reading the book review of 'Drugs and breastfeeding' (Aust Prescr 2004;27:154), I looked up the U.S. product information for metronidazole in the 2000 Physicians' Desk Reference.

The information under Carcinogenicity states that pulmonary tumours were found in all six studies done in mice including one with dosing only every fourth week; there were also malignant tumours in the liver and malignant lymphomas. In rats, there were liver and mammary tumours. And finally, the drug is genotoxic — it damages the DNA directly.

Under the heading Nursing Mothers, it says, "because of the potential for tumorigenicity shown for metronidazole in mouse and rat studies, a decision should be made whether to discontinue nursing or to discontinue the drug." It further states that, "metronidazole is secreted in human milk in concentrations similar to those found in plasma."

Based on this information, I take issue with the reviewer who says that, based on this book, she is reassured that metronidazole "will do the baby no harm." On the contrary, there is tremendous potential for harm and the U.S. product information actually says not to nurse when using metronidazole. So much for the usefulness of this book!

Elizabeth Barbehenn, Ph.D.
Research Analyst
Public Citizen's Health Research Group
Washington DC, USA

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