The full, clinically endorsed recommendation should be obtained from Lareb [link].   

Clinical Overview  

Metronidazole is used in pregnancy to treat infections, including bacterial vaginosis and trichomoniasis, and as perioperative prophylaxis. Because pregnancy alters the activity of drug metabolizing enzymes and foetal exposure occurs, dose-adjustments may be necessary to maintain effective concentrations during pregnancy.  

Pharmacokinetics of metronidazole in pregnancy 

Metronidazole is metabolised by the hepatic enzymes CYP3A4 and CYP2D9, whose activities increase during pregnancy. Limited studies report a shorter half-life and lower plasma concentrations of metronidazole during pregnancy. Placental transfer occurs, with cord concentrations that are slightly lower than maternal concentrations. 

Efficacy and safety of metronidazole in pregnancy 

Evidence indicates metronidazole remains effective for bacterial vaginosis and trichomoniasis, with similar cure rates between single-dose and multiple-dose regimens. The clinical impact of reduced exposure during pregnancy remains uncertain. Safety experience is extensive and largely reassuring, with no consistent increase in congenital malformations.  

Conclusion  

Although plasma concentrations of metronidazole are lower in pregnant individuals compared to those who are not pregnant, the clinical relevance of this reduction remains uncertain. Overall safety evidence is reassuring.