Trichomoniasis and chlamydia are both common, both often silent, and both fully curable — but they're entirely different organisms. Trich is a parasite treated with metronidazole; chlamydia is a bacterium treated with doxycycline. The symptoms overlap too much to tell apart by feel, so a lab test is the only reliable way to know which one you have.
most common curable STI; metronidazole
~1.65M US cases/yr; cured with doxycycline
| Item | Value |
|---|---|
| Trichomoniasis | curable — most common curable STI; metronidazole |
| Chlamydia | curable — ~1.65M US cases/yr; cured with doxycycline |
The bottom-line difference
The core split is what kind of bug causes each. Trichomoniasis comes from a single-celled protozoan parasite, Trichomonas vaginalis, while chlamydia comes from a bacterium, Chlamydia trachomatis CDC. That distinction drives the treatment: trich needs an anti-parasitic from the nitroimidazole family (metronidazole or tinidazole), and chlamydia needs an antibacterial antibiotic (doxycycline). One drug won't reliably treat the other, which is why pinning down which infection you have actually matters.
What each one is
Trichomoniasis
Trichomoniasis is caused by Trichomonas vaginalis, a protozoan parasite that lives in the lower genital tract. It's the most common curable non-viral STI in the US, with an estimated 2.6 million infections, and it falls disproportionately on women, who account for more than 80% of cases Sex Transm Dis, 2018. Because it's a parasite rather than a bacterium or virus, it responds to a specific class of drugs and not to standard antibiotics.
Chlamydia
Chlamydia is caused by the bacterium Chlamydia trachomatis; most genital infections in the US are the serovars labeled D–K. It's the most-reported STI in the country, with about 1.65 million cases reported in 2023 (492 per 100,000), a number that's held roughly flat across recent years CDC AtlasPlus, 2023. Left untreated in women, chlamydia can ascend from the cervix into the upper reproductive tract, which is where the serious complications come from.
Symptoms compared
Both infections are famous for hiding. With trichomoniasis, about 70% of infected people have no signs or symptoms at all. Chlamydia is similarly quiet — roughly three quarters of infected women and half of infected men notice nothing CDC. When symptoms do show up, they look frustratingly alike.
Trichomoniasis tends to cause, in women, itching, burning, redness or soreness of the genitals, discomfort with urination, and a clear, white, yellowish or greenish discharge with a fishy smell. In men it can cause itching or irritation inside the penis, burning after urinating or ejaculating, and some discharge — though men are very often asymptomatic carriers.
Chlamydia in women can cause abnormal vaginal discharge and burning on urination. If it spreads into the upper tract it can bring lower abdominal or low-back pain, fever, pain during intercourse, and bleeding between periods — signs that the infection has moved beyond the cervix and is causing pelvic inflammation.
How to tell them apart
Honestly, you mostly can't — not from symptoms alone. The discharge, the burning, and the irritation overlap enough that even an experienced clinician won't bet on the diagnosis without a test. A few clues lean one way: the classic frothy, greenish, fishy-smelling discharge points more toward trichomoniasis, while bleeding between periods and deep pelvic or low-back pain raise concern for chlamydia spreading upward. But these are soft signals, not proof. The practical takeaway is that a test settles it — guessing and treating blind risks taking the wrong drug entirely.
Side-by-side comparison
| Trichomoniasis | Chlamydia | |
|---|---|---|
| Cause | Protozoan parasite (T. vaginalis) | Bacterium (C. trachomatis) |
| How common | ~2.6 million infections; most common curable non-viral STI | ~1.65 million reported cases in 2023; most-reported STI |
| Often silent? | Yes — about 70% have no symptoms | Yes — ~75% of women, ~50% of men |
| Typical symptoms | Itching, soreness, frothy discharge with a fishy smell | Abnormal discharge, burning urination, possible pelvic pain |
| Best test | NAAT (vaginal swab or female urine) | NAAT (genital and extragenital) |
| First-line treatment | Metronidazole (course for women, single dose for men) | Doxycycline, twice daily for a week |
| Curable? | Yes | Yes |
Testing
For both infections, the nucleic acid amplification test (NAAT) is the preferred method because it detects the organism's genetic material with high accuracy. For trichomoniasis, NAAT (for example the Aptima T. vaginalis assay) runs a sensitivity of about 95–100%, far better than older microscopy, using vaginal swabs or a female urine sample CDC, 2021. For chlamydia, NAAT is the optimal method for both genital and extragenital (throat and rectal) sites CDC STI Guidelines, 2021.
What testing actually looks like: usually a urine sample, a self-collected swab, or a brief exam depending on which infection is suspected — quick and not painful. It's free or low-cost at health departments, Planned Parenthood, and Title X clinics, and many people simply order an at-home or in-lab panel. Timing matters too, since testing too soon after exposure can miss an early infection — see when to test after exposure before you book. You can get tested for both at once. For the full walk-through on the parasite side, see our guide to trichomoniasis testing & diagnosis.
Treatment compared
This is where the parasite-vs-bacterium split really shows. Trichomoniasis in women is treated with metronidazole 500 mg orally twice daily for 7 days; the 2021 guidelines moved women off the old single 2 g dose because the longer course cut retest-positive rates roughly in half — in a randomized trial about 19% of women on the single dose were still infected at follow-up versus 11% on the week-long course Muzny et al.. Men are treated with metronidazole 2 g orally as a single dose, and tinidazole 2 g as a single dose is an alternative for either sex.
Chlamydia is treated with doxycycline 100 mg orally twice daily for 7 days. That replaced the old single-dose azithromycin because doxycycline clears rectal infection far more completely — a randomized trial found a 100% cure with doxycycline versus 74% with azithromycin, which is why doxycycline is now first-line Clin Infect Dis. Partner treatment matters for both: expedited partner therapy, where you carry medication to a partner, measurably cuts repeat infection Golden et al.. After chlamydia treatment, plan to retest a few months later to catch reinfection — more on that in our piece on chlamydia reinfection.
Can you have more than one at once?
Yes. Co-infection is common because both spread the same way and share the same risk factors, so testing positive for one is a reason to be screened for the other (and for gonorrhea and HIV). A NAAT panel can check for several at the same time from a single sample. Having both at once doesn't change the principle of treatment — each infection still needs its own correct drug, since the metronidazole that clears trich does nothing for chlamydia and vice versa.
When to see a clinician
Get checked if you have any new genital symptoms — unusual discharge, burning, itching, pelvic or low-back pain, or bleeding between periods — or if a partner tells you they tested positive, even when you feel completely fine. Because both infections are so often silent, routine screening is the only way many people learn they're infected. Seek care promptly if you develop fever with pelvic pain, which can signal that infection has spread upward.