What STDs Does Metronidazole Treat? Flagyl Facts
Metronidazole (Flagyl) treats the two most common vaginal infections misread as STDs: trichomoniasis and bacterial vaginosis. Standard regimens: 2g single dose or 500mg twice daily ×7 days for trich; 500mg twice daily ×7 days for BV. It cures over 90% of cases — with one non-negotiable rule: zero alcohol during treatment and three days after, or the reaction is genuinely miserable.
What metronidazole actually treats
| Condition | Regimen | Cure rate |
|---|---|---|
| Trichomoniasis (women) | 500mg BID ×7d preferred; 2g×1 alternative | ~92–97% (7-day slightly better) |
| Trichomoniasis (men) | 2g single dose | ~95% |
| Bacterial vaginosis | 500mg BID ×7d | ~80–90% |
| Amoebiasis/giardia | Various courses | High (non-STI uses) |
The drug works through a neat biochemical trick: anaerobic organisms (trich, Gardnerella communities) metabolize it into free-radical forms that shred their own DNA. Aerobic human cells mostly skip this pathway — hence tolerability.
The alcohol rule (not folklore)
Metronidazole blocks alcohol-metabolizing enzymes, causing acetaldehyde buildup — the same mechanism as disulfiram (Antabuse). The resulting reaction within minutes of drinking:
- Flushing and pounding heartbeat
- Severe nausea/vomiting
- Throbbing headache, sweating, dread
Misery lasts hours. The window: no alcohol during the course plus 72 hours after finishing (tinidazole's cousin rule runs longer). Mouthwash and cooking-wine trace amounts don't trigger it; drinks do.
Other side effects worth expecting: metallic taste (classic), nausea taken WITH food minimizes it, dark urine (harmless), occasional headache. Severe neurologic symptoms (numbness, seizures) are rare long-course events needing calls to prescribers.
Partner rules differ by diagnosis
- Trich: partners MUST treat simultaneously — reinfection ping-pong is trich's defining feature (the full cycle).
- BV: male partner treatment unnecessary; female partners in recurrent cases sometimes warrant evaluation.
- No sex during treatment for either condition until course completes — irritation plus cure-timing logic.
- Pregnancy: metronidazole is considered safe after the first trimester and commonly used throughout per current guidance — symptom relief benefits outweigh theoretical risks.
Where metronidazole doesn't reach: chlamydia, gonorrhea, syphilis, anything viral (full drug map). Retest timing after treatment lives in window periods; the BV-vs-trich distinction it treats in BV explained.
Frequently asked questions
Sources & further reading
- 📚 CDC — Trichomoniasis treatment — www.cdc.gov/std/treatment-guidelines/trichomoniasis.htm
- 📚 CDC — BV treatment — www.cdc.gov/bacterial-vaginosis/hcp/treatment
- 📚 MedlinePlus — Metronidazole — medlineplus.gov/druginfo/meds/a689011.html