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 Metronidazole in early pregnancy  

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How safe is metronidazole in pregnancy, especially 1st trimester?

I know it's cat B. Some say it's safe throughout pregnancy. But I'd avoid it during 1st trimester.

What do you think?


Pregnancy Category B. Metronidazole crosses the placental barrier and enters the fetal circulation rapidly. Reproduction studies have been performed in rats at doses up to five times the human dose and have revealed no evidence of impaired fertility or harm to the fetus due to metronidazole. No fetotoxicity was observed when metronidazole was administered orally to pregnant mice at 20 mg/kg/day, approximately one and a half times the most frequently recommended human dose (750 mg/day) based on mg/kg body weight; however in a single small study where the drug was administered intraperitoneally, some intrauterine deaths were observed. The relationship of these findings to the drug is unknown. There are, however, no adequate and well-controlled studies in pregnant women. Because animal reproduction studies are not always predictive of human response, and because metronidazole is a carcinogen in rodents, this drug should be used during pregnancy ONLY if clearly needed.

Use of Flagyl for trichomoniasis during pregnancy should be restricted to those in whom alternative treatment has been inadequate. Use of Flagyl (metronidazole) for trichomoniasis in pregnancy should be carefully evaluated because metronidazole crosses the placental barrier and its effects on the human fetal organogenesis are NOT KNOWN.

Nursing mothers

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, taking into account the importance of the drug to the mother. Metronidazole is secreted in human milk in concentrations similar to those found in plasma.


Thanks !!


kaplan says safe to give in first tri too.


according to blueprints:
Metronidazole ( Flagyl) is the treatment of choice and often used as a vaginal gel to avoid increasing nausea during pregnancy.


Penicillins (category B): These are the most widely used antibiotics in pregnancy because of their wide margin of safety and lack of known toxicity. In the collaborative perinatal project, 3546 women used penicillin during the first trimester and no adverse effects were demonstrated. Ticarcillin, however, has shown some toxicity in animals and may not be safe in pregnancy.

Cephalosporins (category B): This group has not been well studied in the first trimester and should therefore not be considered the first line of treatment in the first trimester of pregnancy. Generally, these drugs are considered safe and have shown no teratogenicity.

Sulfonamides (category C): Avoid sulfonamides in the third trimester of pregnancy and during breastfeeding. Although these agents cause no known damage in utero, they can cause hyperbilirubinemia and kernicterus if the drug is still present in the neonate after birth. In mothers with G-6-PD deficiency, sulfonamide use has been associated with hemolysis. The combination of sulfonamides with trimethoprim in the first trimester has been associated with cardiovascular birth defects.

Tetracyclines (category D): Tetracyclines have identifiable adverse effects in both the mother and the fetus. Pregnant women are susceptible to acute fatty necrosis of the liver, pancreatitis, and renal damage. In the fetus, these agents can cause stunting of growth, discoloration of teeth, and hypoplasia of dental enamel. Although tetracyclines have not proven to be deleterious in the first trimester or in smaller doses, they are best avoided.

Aminoglycosides (category D): Aminoglycosides used in conjunction with hypomagnesemia and hypocalcemia, and in conjunction with calcium channel blockers, may cause neuromuscular blockade. Streptomycin is thus far the only agent in the class with reported toxicity, causing mild congenital deafness only detectable with vestibular testing or with an audiogram.

Nitrofurantoin (category B): The collaborative perinatal project showed no increased risk of anomalies in 590 women who were exposed to the drug. In mothers with G-6-PD deficiency, it has caused hemolysis in both the mother and the fetus and should therefore be avoided near delivery.

Quinolones (category C): Although animal studies have shown arthropathies, no human studies have been conducted and no cases of teratogenicity have been reported. These agents have a high affinity for bone and cartilage.

Metronidazole (category B): Metronidazole should not be used in the first trimester or during lactation. When used in the second or third trimester, large single-dose treatments should be avoided.

Macrolides (category B): These agents have not been associated with birth defects and are considered safe for use in pregnancy.

Clindamycin (category B): This drug has not been associated with birth defects.


You could give Metronidazole TOPICAL but shouldn't give the ORAL Metro during pregnancy... There was a question in UW about this -the pregnant woman had BV and they asked for the treatment and the WRONG answer was Metro because they said it is teratogenic in the first trimester. The RIGHT answer was Clindamycin cream.


i wonder why KLN actually says that it is safe during pregnancy, including first trimester.
(p 140 in 2005/6 edition, trichomonas management)


I can recommend a reputable pharmacy (Flagyl) I recieved my pills within one week of ordering them.
P.S. 5% discount coupon code: 4y3g6f4k

Edited by sarah4343 on Feb 07, 2016 - 7:30 AM


UW says NO, but clinically doctors prescribe it.

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