Bacterial Vaginosis

Clinical Snapshot

Bacterial vaginosis (BV) results from a shift in the normal vaginal flora: the lactobacilli that normally dominate are replaced by an overgrowth of anaerobic organisms, chiefly Gardnerella vaginalis along with Mycoplasma species and other anaerobes. BV is not itself classified as a sexually transmitted infection, and — unlike trichomoniasis — male sexual partners are not treated, since partner treatment does not reduce recurrence. Treatment is reserved for symptomatic patients and a specific set of asymptomatic situations where treatment is protective.^[1]

Recognize the Condition

Typical Presentation

Notably different from the other 2 discharge-causing infections: no pruritus, no inflammation.

Thin, grey or milky, copious vaginal discharge.
A distinctive fishy odour, often more noticeable after intercourse.
No significant pruritus and no inflammation — their presence should make you reconsider the diagnosis.

Risk Factors

New or multiple sexual partners, and a new sexual relationship generally — though BV is not classified as an STI itself.
Vaginal douching.
Absence of condom use (some association, though causality is less clear-cut than for classic STIs).
Lack of vaginal lactobacilli for any reason, including recent antibiotic use.
Recurrence is common, and a clear single cause for recurrent BV is often not identifiable.

Red Flags & Refer

Refer or escalate for any of the following:

Pelvic pain, fever, or signs suggesting pelvic inflammatory disease rather than uncomplicated BV.
Recurrent BV (3 or more episodes per year) that does not respond to an extended treatment course — maintenance suppressive therapy or further gynecologic assessment may be needed.

Assessment & Diagnosis

Differentiating the 3 vaginal discharge infections

Parameter	Vulvovaginal candidiasis	Trichomoniasis	Bacterial vaginosis
Pruritus	Present	Present	Absent
Odour	Absent	Present	Present (fishy)
Discharge	White, clumpy and curdy	Off-white or yellow, frothy	Grey or milky, thin, copious
Inflammation	Present	Present	Absent
Vaginal pH	Under 4.5 (normal)	Over 4.5	Over 4.5
"Whiff" test (KOH)	Negative	Negative	Positive
Microscopy, specific finding	Budding yeast, pseudohyphae	Motile trichomonads	Clue cells; predominant gram-negative curved bacilli and coccobacilli
Microscopy, PMNs	Increased	Markedly increased	Not increased
Microscopy, lactobacilli	Present	Absent	Absent

Nonsexually transmitted causes of vaginitis are also common and include atrophic vaginitis, trauma, contact or irritant dermatitis (detergents, contraceptive chemicals), foreign bodies, and allergy (e.g., latex condoms). Candidiasis and bacterial vaginosis are not generally considered sexually transmitted; trichomoniasis is.

Simple bedside diagnosis

Vaginal pH over 4.5 — check with pH paper; a normal pH under 4.5 argues against BV.
Positive "whiff" test — a fishy odour on mixing a sample with 10% potassium hydroxide (KOH).
Clue cells on microscopy — vaginal epithelial cells covered with numerous coccobacilli, seen on a saline wet mount.

Amsel's clinical criteria (discharge appearance, pH over 4.5, positive whiff test, and clue cells — 3 of 4 required) or a Gram-stain-based scoring method are both acceptable diagnostic approaches; use whichever your setting supports. NAAT-based testing is increasingly available and can support the diagnosis in more equivocal cases.^[1]

Treatment Targets

Resolve discharge, odour, and any associated discomfort in symptomatic patients.
Prevent BV-related complications before IUD insertion, gynecologic surgery, therapeutic abortion, or upper genital tract instrumentation.
Reduce the risk of preterm birth in pregnant patients at high risk who are also symptomatic or otherwise meet treatment criteria.

Choose Initial Treatment

Direct treatment rule

Treat every symptomatic patient.

Do not routinely treat asymptomatic BV — treatment has not been shown to improve outcomes in an asymptomatic patient who is not in one of the specific situations below.

Treat asymptomatic BV only before IUD insertion, gynecologic surgery, therapeutic abortion, or upper genital tract instrumentation, or in a pregnant patient at high risk of preterm delivery (previous preterm labour or premature rupture of membranes).^[1]

Do not treat

Male sexual partners are not treated — this does not reduce recurrence risk and is not part of BV management, unlike trichomoniasis.

Non-Pharmacologic Treatment

Reduce or avoid vaginal douching, which disrupts normal flora.
Consistent condom use may modestly reduce recurrence risk, although evidence for this is not as strong as for classic STIs.
No partner treatment or partner notification is indicated.

Pharmacologic Treatment Options

Drug / intervention	Role	Dose / use	Important adverse effects	Teaching points
Metronidazole, oral Preferred	Systemic BV treatment	500 mg PO BID for 7 days	GI upset, metallic taste, nausea	Useful when systemic therapy is preferred; review alcohol counselling and interactions.
Metronidazole 0.75% vaginal gel Preferred	Local BV treatment	5 g intravaginally once daily for 5 days	Local irritation, discharge, candidiasis	Not a treatment for trichomoniasis.
Clindamycin 2% vaginal cream Preferred	Local BV treatment	5 g intravaginally at bedtime for 7 days	Vulvovaginal irritation; diarrhea is uncommon	Oil-based formulation may weaken latex barrier products for the interval stated in the product monograph.
Clindamycin, oral Alternative	Systemic alternative	300 mg PO BID for 7 days	Diarrhea; C. difficile-associated disease	Use when an oral alternative is required.

Recurrent BV (3 or more episodes per year) needs an extended-duration course rather than the standard duration — see Drug Tables for the specific extended regimens.^[1]

Special Populations

Pregnancy: screen for BV specifically in high-risk pregnancies (previous preterm labour or premature rupture of membranes) or when the patient is symptomatic. In lower-risk, asymptomatic pregnancies, treatment has not been shown to improve pregnancy outcomes, so routine screening is not indicated. When treatment is given in pregnancy, use a systemic (oral) regimen — metronidazole or clindamycin — rather than a topical one, since topical therapy is specifically not recommended when the treatment goal is preventing adverse pregnancy outcomes in a patient at increased preterm-birth risk.^[2]

Breastfeeding: it is not necessary to withhold breastfeeding during metronidazole treatment; some clinicians advise against breastfeeding for 12–24 hours specifically after a 2 g single dose.

Monitoring & Follow-Up

Retest 1 month after completing oral metronidazole therapy to confirm treatment success — recurrence within the first few months of treatment is common enough that this follow-up is worthwhile.
Reassess promptly if odour or discharge returns before 1 month, rather than waiting for the scheduled follow-up.
For recurrent BV, look for and address contributing factors, and consider extended or maintenance therapy (see Drug Tables).

Treatment Failure / Next Step

Confirm the regimen was completed correctly, including full duration for topical therapy.
Re-examine and retest to confirm the diagnosis is still BV and not a different or superimposed cause of discharge.
For a single recurrence, retreat with the same or an alternative first-line regimen.
For recurrent BV (3 or more episodes per year), move to an extended-duration or maintenance regimen (see Drug Tables) and address contributing factors such as douching.
Refer for gynecologic assessment if recurrence continues despite an adequate extended regimen.

Common Errors to Avoid

Treating asymptomatic BV outside the specific indicated situations (pre-procedure, high-risk pregnancy).
Treating the male sexual partner — this is not indicated and does not reduce recurrence.
Using topical therapy in a high-risk pregnant patient when the goal is preventing preterm birth — use a systemic regimen instead.
Attributing persistent pruritus or inflammation to BV — these findings point toward candidiasis or trichomoniasis instead.

Clinical Decision Algorithm

Bacterial vaginosis: who to treat and how

Confirm BV: vaginal pH over 4.5, positive whiff test, clue cells on microscopy

Symptomatic

First episode this year (under 3): metronidazole (oral or vaginal gel) or topical clindamycin, standard duration

3 or more episodes this year: extended metronidazole 500 mg PO BID x 10-14 days, or vaginal-gel maintenance (10 days then twice weekly x 4-6 months); investigate contributing factors

Asymptomatic

Check for a specific treatment indication: pre-IUD insertion, pre-gynecologic surgery, pre-therapeutic abortion, pre-upper-tract instrumentation, or high-risk pregnancy

One of these situations applies

Treat — oral metronidazole 500 mg BID x 7 days (use oral, not topical, if the indication is high-risk pregnancy)

None apply

No treatment indicated — treatment has not been shown to benefit an asymptomatic patient outside these situations

Drug Tables

How to use this table: Doses are adult doses and require patient-specific confirmation, including renal/hepatic adjustment. Male partners are not treated. Select any drug name to open its complete Pharmasist Drug Database page.

Nitroimidazoles

Both oral and topical metronidazole are preferred first-line options.

Drug	Dosage	Adverse effects	Comments
Metronidazole, oral (standard course) Flagyl, generics	500 mg PO BID x 7 days.	GI upset, metallic taste, urethral burning, dark or reddish-brown urine discolouration.	Avoid alcohol during treatment and for the product-specific interval afterward — disulfiram-like reaction. Can be used in pregnancy and breastfeeding; preferred over topical therapy when preventing adverse pregnancy outcomes is the goal.
Metronidazole, oral (single-dose alternative) Flagyl, generics	2 g PO as a single dose.	Same as above.	Associated with a higher failure rate than the 7-day course — use the standard course when adherence is not a major concern.
Metronidazole, oral (recurrent BV, 3+ episodes/year) Flagyl, generics	500 mg PO BID x 10-14 days.	Same as above.	Extended-duration course for recurrent disease.
Metronidazole, vaginal gel 0.75% NidaGel	1 applicatorful (5 g) PV once daily x 5 days.	Vaginal discharge, vulvovaginal irritation, pain and discomfort; may promote yeast overgrowth.	Similar cure rate to oral metronidazole. Not recommended when the goal is preventing adverse pregnancy outcomes in a high-risk pregnancy — use an oral regimen instead in that situation.
Metronidazole, vaginal gel (recurrent BV, 3+ episodes/year) NidaGel	1 applicatorful (5 g) PV daily x 10 days, then 5 g PV twice weekly for 4-6 months.	Same as above.	Maintenance regimen for recurrent disease.

Lincosamides

Preferred topical alternative to metronidazole; oral clindamycin is a systemic alternative.

Drug	Dosage	Adverse effects	Comments
Clindamycin, vaginal cream 2% Dalacin Vaginal Cream	1 applicatorful (5 g) PV once daily x 7 days.	Vulvovaginitis.	Delay treatment until the menstrual period has finished. Contains mineral oil, which may decrease the effectiveness of latex condoms and diaphragms — advise an alternative barrier method or abstinence during treatment. Not recommended when preventing adverse pregnancy outcomes is the goal — use an oral regimen instead.
Clindamycin, oral Dalacin C, generics	300 mg PO BID x 7 days.	GI upset; rare Clostridioides difficile colitis, diarrhea.	An alternative regimen when topical therapy is not wanted. Can be used in pregnancy and breastfeeding; retest 1 month after completion to confirm cure.

Evidence note: Drug inventory, dosing, adverse effects, and comments reflect standard Canadian practice per current PHAC guidance.^[1]

References

Sexually transmitted and blood-borne infections: Guides for health professionals — Public Health Agency of Canada. Canadian Guidelines on Sexually Transmitted Infections
Screening and management of bacterial vaginosis in pregnancy — Journal of Obstetrics and Gynaecology Canada (Yudin MH, Money DM; Infectious Diseases Committee). J Obstet Gynaecol Can 2008;30(8):702-716

Professional reference only. Apply clinical judgment and verify patient-specific decisions against current authoritative guidance and product information.