Urogenital Trichomoniasis: Symptoms, Testing and the Metronidazole Course That Cures It
Trichomonas vaginalis is a single-celled parasite with four whipping flagella that propel it across the lining of the genital tract. It is the most common curable sexually transmitted infection in the world, more common than chlamydia and gonorrhoea combined, and the one people have heard of least.
Most infections cause nothing. Roughly 70 to 85 percent of people carrying it have no symptoms at all, and in men that figure is higher still. The parasite can persist for months or years in a silent carrier who keeps passing it on, which is why trichomoniasis is not usually caught early and is very often caught twice.
Two things make it worth taking seriously despite the mild symptoms. It roughly doubles the risk of acquiring HIV, by inflaming the tissue and recruiting the very cells the virus infects. And in pregnancy it raises the risk of preterm birth and low birth weight.
👩 What it looks like in women
When symptoms do appear, they usually start five to twenty-eight days after exposure.
- 💧 Discharge, classically yellow-green and frothy, though in practice it is often just increased and thin
- 👃 A strong odour, sometimes described as musty
- 🔥 Itching, soreness and redness of the vulva and vagina
- 🚽 Burning on urinating, and needing to go more often
- 🛏️ Pain during sex, and sometimes bleeding afterwards
- 🐞 Lower abdominal discomfort in some cases
🍓 The strawberry cervix. Tiny haemorrhages on the cervix give a speckled red appearance known as colpitis macularis. It is highly characteristic of trichomoniasis, and it is visible to the naked eye in only about 2 percent of cases, which is a good illustration of why the diagnosis is not made by looking.
👨 What it looks like in men
Usually nothing. Most infected men clear it spontaneously within a few weeks, but during that time they transmit it, and a proportion carry it far longer. Where symptoms occur they are easily dismissed:
- 💧 Thin discharge from the urethra, often only in the morning
- 🔥 Burning after urinating or after ejaculation
- 😟 Irritation or itching inside the penis
- 🪲 Occasionally prostatitis or epididymitis with pelvic ache
Men are frequently the reason an infection keeps coming back in a partner who has been treated correctly twice.
🧪 Testing: the method matters
Symptoms overlap almost completely with bacterial vaginosis and thrush, so the diagnosis cannot be made on appearance. The tests differ substantially in how good they are.
| Test | Sensitivity | In practice |
|---|---|---|
| NAAT, molecular testing | Over 95 percent | The gold standard. Works on a vaginal swab, urine or a urethral swab, and detects infection in men, where other tests fail |
| Culture | 75 to 95 percent | Accurate but takes up to a week |
| Rapid antigen test | 80 to 90 percent | Results in minutes; useful where molecular testing is unavailable |
| Wet mount microscopy | Only 50 to 65 percent | Cheap and immediate, but misses a third to a half of infections. The organism stops moving as the slide cools, so it must be read within minutes |
ℹ️ A negative microscopy result does not exclude trichomoniasis. If symptoms persist after a normal wet mount, ask whether molecular testing was done. This single point explains a large share of cases treated repeatedly for thrush that were never thrush.
Because sexually transmitted infections travel together, testing for chlamydia, gonorrhoea, HIV and syphilis is done at the same time. Bacterial vaginosis frequently coexists.
💊 Treatment: the dose depends on who you are
Metronidazole, available here as Flagyl (Metronidazole 200/400/500 mg), enters the parasite, is chemically reduced inside it into reactive compounds, and destroys its DNA. Human cells do not perform that reduction, which is why the drug is selective.
The important development of recent years is that the regimen is no longer the same for everyone.
| Who | Regimen | Why |
|---|---|---|
| Women | 500 mg twice daily for 7 days | A randomised trial found the seven-day course reduced treatment failure by roughly half compared with a single 2 g dose. It is now the preferred regimen |
| Men | 2 g as a single dose | Remains effective in men, and a single supervised dose solves the adherence problem |
| People with HIV | 500 mg twice daily for 7 days, whatever their sex | Higher failure rates with single dosing |
| In pregnancy | Treated, not postponed | Metronidazole has not been shown to cause birth defects. The older advice to avoid it in the first trimester no longer stands, and untreated infection carries its own risk |
🍷 No alcohol during the course or for three days afterwards. Metronidazole blocks the enzyme that clears acetaldehyde, producing a disulfiram-like reaction: flushing, throbbing headache, violent nausea and vomiting, racing heart and a fall in blood pressure. It can be severe. This includes beer, wine and spirits, and also alcohol-containing mouthwash and cough syrup, and preparations containing propylene glycol.
⚠️ Other things to know while taking it
- 👄 A metallic taste is almost universal and harmless
- 🤢 Nausea, loss of appetite and headache are common; taking it with food helps
- 🟫 Urine may darken, which is expected
- 🩸 It raises INR on warfarin, so monitoring is increased
- ⚡ Long or repeated courses can cause tingling or numbness in the hands and feet, which should be reported
- 🚫 It interacts with disulfiram and lithium and is avoided in the first trimester only where an alternative exists and treatment can safely wait, which it usually cannot
👫 The part that decides whether it comes back
Treating one person cures one person. Trichomoniasis returns in a large share of cases for one reason, and it is not drug failure.
- All sexual partners from the last 60 days are treated, whether or not they have symptoms and whether or not they test positive. Where partners cannot attend, many services provide treatment for them directly
- No sex until seven days after both partners have finished treatment and symptoms have resolved. Not after the first dose, and not after feeling better
- A repeat test three months later is recommended for women, because reinfection rates reach roughly 17 percent within that window. This is a test for reinfection, not a test of cure
🔁 If symptoms return after treatment, the order of likelihood is: reinfection from an untreated partner, then a different condition causing similar symptoms, then genuine drug resistance. Low-level metronidazole resistance exists in roughly 4 to 10 percent of strains. It is managed with a higher dose or with tinidazole, a related drug with a longer half-life, which we do not currently stock. Persistent cases need specialist referral and susceptibility testing rather than another identical course.
🔍 What else causes the same symptoms
| Condition | Discharge | Distinguishing feature |
|---|---|---|
| Trichomoniasis | Yellow-green, sometimes frothy, increased | Soreness and burning; often a partner with symptoms |
| Bacterial vaginosis | Thin, grey-white | Fishy odour, stronger after sex; little or no itching |
| Thrush | Thick, white, curd-like | Intense itching; no odour. Treated with Diflucan (Fluconazole) |
| Chlamydia or gonorrhoea | Variable, may be absent | Often silent; bleeding between periods, pelvic pain |
| Irritant reaction | Absent or minimal | Follows a new soap, gel, lubricant or washing powder |
Treating the wrong one is common. Repeated self-treatment with antifungal creams for what turns out to be trichomoniasis wastes months and leaves the partner untreated the whole time.
🛡️ Prevention that works
- Condoms
- Reduce transmission substantially, though not completely, because the parasite can occupy areas a condom does not cover.
- Screening rather than waiting for symptoms
- Given that most infections are silent, testing at a change of partner is more useful than testing when something feels wrong.
- Do not douche
- It disturbs the protective vaginal flora and is associated with higher rates of infection, not lower.
- Treat partners simultaneously
- The single most effective step against recurrence.
- Annual screening in HIV-positive women
- Recommended because of higher prevalence, higher failure rates and the effect on HIV transmission.
📞 When to seek care promptly
- Fever with pelvic pain, which may indicate pelvic inflammatory disease
- Symptoms during pregnancy, at any stage
- Symptoms that persist after a completed course
- A partner diagnosed with any sexually transmitted infection, even with no symptoms of your own
- Recurrent discharge treated repeatedly as thrush without improvement
- Tingling or numbness in hands or feet while taking metronidazole
❓ Frequently asked questions
What is the metronidazole dose for trichomoniasis?
For women, 500 mg twice daily for seven days, which is now preferred because it roughly halves the failure rate compared with a single dose. For men, a single 2 g dose. People with HIV take the seven-day course regardless of sex.
Can I drink alcohol while taking metronidazole?
No, and not for three days after finishing. The combination causes a disulfiram-like reaction with flushing, severe nausea and vomiting, headache, racing heart and a drop in blood pressure. This includes alcohol-containing mouthwash and cough syrups.
Can men get trichomoniasis?
Yes, and usually without symptoms. Many men clear it within weeks but transmit it in the meantime, and some carry it far longer. An untreated male partner is the most common reason an infection keeps returning in a woman who has been treated correctly.
Does my partner need treatment if they have no symptoms?
Yes. All partners from the last 60 days are treated regardless of symptoms or test results, and sex is avoided until seven days after both have finished treatment. Without this the infection simply passes back.
Why did my test come back negative when I have symptoms?
Probably because it was wet mount microscopy, which misses a third to a half of infections. Molecular NAAT testing detects over 95 percent and works on urine as well as swabs. Ask which test was used.
Is metronidazole safe in pregnancy?
It has not been shown to cause birth defects, and the older advice to avoid it in the first trimester no longer stands. Untreated trichomoniasis raises the risk of preterm birth and low birth weight, so treatment is given rather than postponed.
How is trichomoniasis different from bacterial vaginosis or thrush?
Bacterial vaginosis gives thin grey discharge with a fishy odour and little itching. Thrush gives thick white discharge with intense itching and no odour. Trichomoniasis gives yellow-green discharge with soreness and burning. They overlap enough that testing is required.
Why should I be retested after three months?
Because reinfection reaches around 17 percent within three months, usually from an untreated partner. The repeat test looks for reinfection rather than checking that the original treatment worked.
What if the symptoms come back after treatment?
Most often reinfection from an untreated partner, then a different condition, then genuine resistance, which affects roughly 4 to 10 percent of strains at a low level. Persistent cases need specialist assessment rather than another identical course.
Does trichomoniasis affect HIV risk?
Yes. It roughly doubles the risk of acquiring HIV by inflaming the genital tissue and drawing in the immune cells the virus targets, which is one of the main reasons it is treated even when symptoms are mild.
📑 Sources and editorial
- Sexually transmitted infections treatment guidelines covering trichomoniasis regimens, partner management and retesting intervals
- The randomised trial comparing seven-day metronidazole with single-dose therapy in women
- Prescribing information for metronidazole, including the alcohol interaction, warfarin effect and neuropathy with prolonged use
- Comparative studies of NAAT, culture, rapid antigen testing and wet mount sensitivity
- Research on trichomoniasis and HIV acquisition, and on adverse pregnancy outcomes
- Reports on metronidazole resistance rates and tinidazole as an alternative
- Related products: Flagyl (Metronidazole), Diflucan (Fluconazole), antibiotics category
- RXshop Editorial Team — reviewed by Anita Sharma, MD, FACOG — Obstetrician-Gynecologist & Women's Health Specialist
Medical Disclaimer: The information in this article is for educational and informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek guidance from a qualified healthcare provider with any questions you may have regarding a medical condition, and before starting, stopping or changing any medication.