Metronidazole (Flagyl®)

STI Standards

Midwives may independently prescribe, order, and administer drugs in the community, hospital or other sites of midwifery practice to treat sexually transmitted infections for clients in case, as designated under specialized practice certification. Midwives without specialized practice certification in sexually transmitted infections management are required to refer their clients to an appropriate health care practitioner for treatment. 

An oral antibiotic classified as an antiprotozoal with bactericidal, amebicidal and trichomonacidal action. Readily taken up by anaerobic organisms causing disruption of DNA helical structure and inhibition of protein synthesis and cell death.

Indications and Clinical Use:

Used for the treatment of Bacterial Vaginosis (BV) in pregnant or non-pregnant women. The Centre for Disease Control (CDC) recommends treatment of BV in all symptomatic pregnant women. Asymptomatic pregnant women at high risk for premature labour should be screened for BV and treated. Also used for the treatment of Trichomonas Vaginalis (please refer to write up on Trichomonas Vaginalis). The goal is to reduce the risk of preterm prelabour rupture of the membranes and low birth weight. BV during pregnancy is associated with premature rupture of the membranes, chorioamnionitis, preterm labour, preterm birth and post-cesarean delivery endometritis.

If symptoms persist after the prescribed course of treatment, a consultation with a physician is required.


Hypersensitivity to metronidazole or other nitroimidazole derivatives.
Dose adjustments required in hepatic impairment and end stage renal disease.

Alcohol – Avoid alcohol for 12 hours before, while on drug therapy and for 24-48 hours after finishing. Drug therapy can cause headache, abdominal cramps, nausea, vomiting, flushing, and/or sweating (disulfiram like reaction).

Warnings and Precautions:

Use with caution in those with CNS disease; may cause seizures, encephalopathy, peripheral neuropathy (which may be characterized as numbness or paresthesia of an extremity), uticaria, pruritus, rash, flushing, nasal congestion, fever and/or transient joint pain. Metronidazole can also prolong the QTc interval.

To reduce the incidence of drug-resistant bacteria, use only for the treatment of confirmed infections. Candidiasis may present during metronidazole therapy, treatment with an appropriate antifungal is recommended.


Current data does not suggest Metronidazole poses an increased risk of anomalies when used in pregnancy or have other harmful effects on the fetus.


Limited Data – Probably Compatible – Although the relative infant dose is > 10% this medication is considered suitable in lactation as numerous reports have found few adverse effects in breastfed infants. This medication is also considered suitable for use in both the neonatal and pediatric population.

Adverse Reactions:

Headache, dizziness, changes in vision, dry mouth, metallic taste, furry tongue, glossitis, nausea, vomiting, diarrhea or constipation, abdominal pain, anorexia, changes in liver function, darkening of the urine and/or rash.

Drug Interactions
E.g. Alcohol, Lithium, Aripiprazole, certain SSRIs, Mebendazole, Warfarin, Phenytoin, or
QTc prolonging medications. For more information, please consult drug interaction references.

Dosage and Administration:

Bacterial Vaginosis (BV)
500 mg orally twice daily for 7 days; or
250 mg orally3 times daily for 7 days

Alternate treatment – Metronidazole gel 0.75%, one applicator (5g) once a day intravaginally for 5 days.

With regular tablets or capsules, food decreases the rate of absorption and peak plasma concentrations. With extended release tablets, food increases rate of absorption and peak plasma concentrations. Food does not change the total amount of drug absorbed.

Metronidazole (Flagyl®): 2 g orally in a single dose or 500 mg orally every 12 hours (bid) for 7 days.

Onset of Action:

Peak plasma concentrations: 1-3 hours following dose.


Approximately 30–60% of an oral Metronidazole dose is metabolized in the liver by hydroxylation, side-chain oxidation, and glucuronide conjugation. The major metabolite, 2- hydroxy Metronidazole, has some antibacterial and antiprotozoal activity.

Duration of Action:

Half-life elimination: 6-8 hours
Excretion: Urine (60-80%), Feces (6-15%)