Trichomoniasis (often called “trich”) is a sexually transmitted infection caused by a microscopic parasite, not a bacterium or virus. It is the most common curable STI in the world, with an estimated 156 million new cases per year among people aged 15 to 49. Despite how widespread it is, about 70% of infected people never develop symptoms, which means many carry and spread the parasite without knowing it.
What Causes Trichomoniasis
The infection is caused by a single-celled parasite called Trichomonas vaginalis. It’s a protozoan, the same broad category of organism that causes malaria, though it behaves very differently. The parasite is tiny, roughly 7 to 30 micrometers long (far too small to see with the naked eye), and it uses whip-like tails called flagella to move around. In women, it lives in the lower genital tract. In men, it lives in the urethra and prostate. Once it settles in, it reproduces by splitting itself in two, and the infection persists until it’s treated.
Trich spreads through sexual contact, specifically genital-to-genital contact. It passes from penis to vagina, vagina to penis, or vagina to vagina. The parasite doesn’t survive well outside the body, so you can’t catch it from toilet seats, swimming pools, or casual contact.
Symptoms in Women
When symptoms do appear, they tend to show up within 5 to 28 days of exposure, though some people develop signs much later. Women are more likely to experience noticeable symptoms than men. Common signs include:
- Unusual discharge: thin, increased in volume, and sometimes clear, white, yellowish, or greenish with a fishy smell
- Genital irritation: itching, burning, redness, or soreness around the vulva and vaginal area
- Discomfort when urinating
These symptoms can overlap with bacterial vaginosis or yeast infections, which is why testing matters. Relying on symptoms alone often leads to the wrong self-diagnosis.
Symptoms in Men
Men with trich are even less likely to notice anything wrong. When symptoms do occur, they’re typically mild: itching or irritation inside the penis, a burning sensation after urinating or ejaculating, and occasionally a small amount of discharge. Because the symptoms are so subtle, men frequently pass the infection to partners without realizing they’re carrying it. Without treatment, the infection can persist for weeks to months.
How Trichomoniasis Is Diagnosed
There are a few ways to test for trich, and the method your provider uses makes a real difference in accuracy. The gold standard is a nucleic acid amplification test (NAAT), which detects the parasite’s genetic material. NAATs are 95 to 100% sensitive, meaning they catch nearly every case. This test can be run on a vaginal swab or urine sample.
An older method, wet mount microscopy, involves looking at a sample under a microscope for moving parasites. It’s fast and cheap, but it only catches 50 to 66% of infections in women and performs even less reliably in men. If you get a negative result from a microscopy test but still have symptoms, it’s worth asking about a NAAT.
Culture-based testing falls in between, picking up 75 to 96% of infections in women. It’s more accurate than microscopy but slower, often requiring several days for results.
Treatment and What to Expect
Trichomoniasis is fully curable with prescription antibiotics. The treatment differs slightly depending on sex. Women typically take a week-long course of metronidazole, twice daily for seven days. Men usually receive a single, larger dose of the same medication taken all at once. An alternative drug, tinidazole, is sometimes prescribed as a one-time dose for either sex.
One important rule during treatment: avoid all alcohol while taking metronidazole and for at least two full days after finishing it. The combination can cause severe nausea, vomiting, and flushing. This isn’t a minor interaction; it will make you genuinely miserable.
Both partners need to be treated at the same time, even if one has no symptoms. Otherwise the untreated partner simply passes the parasite back, creating a cycle of reinfection. You should also avoid sex until both of you have completed treatment and symptoms have cleared. Retesting is recommended about three months after treatment, because reinfection rates are high.
Why Trich Matters Beyond Symptoms
Because so many cases produce no symptoms, it’s easy to dismiss trich as a minor infection. But untreated trichomoniasis carries real consequences. In women, it can cause inflammation of the genital tract that increases vulnerability to other sexually transmitted infections, including HIV. The parasite damages the protective lining of the genital tract, creating entry points for other pathogens.
During pregnancy, untreated trich is linked to premature delivery and low birth weight. The infection can also make other reproductive health problems worse over time, even in the absence of obvious symptoms.
How Common It Really Is
Globally, the World Health Organization estimated 156 million new trichomoniasis infections in 2020 among adults aged 15 to 49. That total split to roughly 82.6 million cases in males and 73.7 million in females. About one third of all new infections occur in Africa, followed by the Americas. These numbers make trich far more common than chlamydia or gonorrhea, yet it receives a fraction of the public attention. Many routine STI panels don’t even include a trich test unless you specifically request one, which partly explains why it circulates so widely under the radar.
Reducing Your Risk
Condoms significantly reduce the risk of transmission but don’t eliminate it entirely, because the parasite can infect areas of genital skin not covered by a condom. Limiting your number of sexual partners lowers your odds of exposure. If you or a partner have been diagnosed, completing the full course of treatment and waiting to resume sex until both partners are cleared is the most reliable way to break the chain of transmission. Since most cases are asymptomatic, periodic STI screening that includes a trich test is the only way to catch an infection you can’t feel.