What causes foul vaginal odor after intercourse and how is it treated?

Foul Vaginal Odor After Intercourse

Foul vaginal odor after intercourse is most commonly caused by bacterial vaginosis (BV), which should be treated with metronidazole 500 mg orally twice daily for 7 days if symptomatic. 1

Primary Causes

The two main infectious causes of foul vaginal odor after intercourse are:

• Bacterial vaginosis (BV) - the most prevalent cause of vaginal malodor, characterized by a "fishy" odor that often becomes more noticeable after intercourse due to the alkaline pH of semen interacting with vaginal discharge 2, 1
• Trichomoniasis - presents with yellow-green discharge and malodor, though less common than BV 1, 3

BV results from replacement of normal hydrogen peroxide-producing lactobacilli with anaerobic bacteria (Bacteroides, Mobiluncus), Gardnerella vaginalis, and Mycoplasma hominis 2. The characteristic fishy odor intensifies after intercourse because semen's alkaline pH triggers release of volatile amines from the abnormal bacterial flora 2.

Diagnostic Approach

Diagnosis requires three of the following four Amsel criteria: 2, 1

• Homogeneous, white, noninflammatory discharge adhering to vaginal walls
• Clue cells on microscopic examination
• Vaginal pH >4.5 (measured with narrow-range pH paper)
• Positive "whiff test" - fishy odor before or after adding 10% KOH to discharge

Key diagnostic steps: 1, 3

• Measure vaginal pH: >4.5 suggests BV or trichomoniasis; ≤4.5 suggests candidiasis
• Perform saline wet mount to identify clue cells (BV) or motile trichomonads (trichomoniasis)
• Perform KOH preparation for whiff test and to identify yeast/pseudohyphae if candidiasis suspected

Important caveat: Culture for G. vaginalis is not recommended as it lacks specificity - this organism can be isolated from half of normal women 2.

Treatment

For Bacterial Vaginosis (Symptomatic)

Recommended regimen: 2, 1

• Metronidazole 500 mg orally twice daily for 7 days
• Patients must avoid alcohol during treatment and for 24 hours after completion

Alternative regimen: 2

• Metronidazole 2 g orally as single dose (though 7-day regimen preferred)

Critical point: Only symptomatic women require treatment, as the goal is to relieve vaginal symptoms 2. Treatment of male partners is not beneficial and does not prevent recurrence 2, 1.

For Trichomoniasis

Treatment: 1, 4

• Metronidazole 2 g orally as single dose
• Sexual partners must be treated to prevent reinfection (unlike BV)

Tinidazole is an alternative option with cure rates of 80-100% for trichomoniasis 5.

For Vulvovaginal Candidiasis (if present)

While candidiasis typically presents with itching rather than odor, if diagnosed: 1, 4

• Fluconazole 150 mg orally as single dose, OR
• Intravaginal azoles (clotrimazole, miconazole, terconazole)

Special Considerations

Before invasive procedures: 2

• Consider treating BV (even if asymptomatic) before surgical abortion, as it substantially reduces post-abortion pelvic inflammatory disease risk
• BV flora have been recovered from the endometrium and salpinx of women with PID 2

Common pitfalls to avoid: 1, 6

• Do not douche - this disrupts normal vaginal flora and increases infection risk
• Do not treat asymptomatic Candida colonization - present in 10-20% of women without requiring treatment
• Do not culture G. vaginalis - lacks diagnostic specificity

Follow-Up

Return for evaluation only if: 4, 6

• Symptoms persist after treatment
• Symptoms recur within 2 months

For recurrent infections, reassess the diagnosis, consider longer treatment courses, and evaluate for predisposing factors 4.

In approximately one-third of women presenting with genital malodor, no infectious cause is identified 7. If standard workup is negative and symptoms persist, consider noninfectious causes including atrophic vaginitis, irritant/allergic vaginitis, or nonvaginal sources 3, 7.