Mycoplasma genitalium (Mgen) is a sexually transmitted bacterium that causes urethritis in men and cervicitis in women, though it's often silent, especially in women. It's frequently mistaken for chlamydia, has no cell wall (so penicillins don't touch it), and increasingly resists the standard antibiotic, so resistance-guided treatment now matters.
rising — guides therapy
doxy → moxifloxacin
| Item | Value |
|---|---|
| US adults infected | ~1–2% |
| Azithromycin resistance | ~50% — rising — guides therapy |
| Often | no symptoms |
| Treatment | resistance-guided — doxy → moxifloxacin |
What is Mycoplasma genitalium?
Mycoplasma genitalium is one of the smallest known bacteria, and its defining quirk is that it has no cell wall. That single fact shapes everything about treating it. Beta-lactam antibiotics — penicillins and cephalosporins — kill bacteria by attacking the cell wall, so they simply don't work here CDC, 2021. It's an emerging cause of two common syndromes clinicians see all the time: non-gonococcal urethritis (inflammation of the urethra not caused by gonorrhea) in men, and cervicitis (inflammation of the cervix) in women.
For years Mgen flew under the radar because there was no commercial test and its symptoms overlap almost exactly with chlamydia. Many people treated for "chlamydia that won't clear" actually had Mgen all along. The central problem now is resistance. Mutations that defeat the macrolide antibiotic azithromycin have spread widely, so this infection demands a more deliberate approach than the old single-pill cure. For the bigger picture on why detection lags, see why awareness is key in mg diagnosis and treatment.
Symptoms — and the silent reality
Mgen is frequently asymptomatic, particularly in women, so plenty of people carry and pass it without ever feeling sick. When symptoms do appear, they're easy to confuse with other STIs.
In men, Mgen typically shows up as non-gonococcal urethritis: a discharge from the penis and dysuria (a burning or stinging sensation when you urinate). What sets Mgen apart clinically is its tendency to be persistent or recurrent. Urethritis that keeps coming back after a chlamydia regimen is a classic red flag.
In women, the infection more often settles in the cervix as cervicitis, which may cause abnormal discharge, bleeding between periods or after sex, or no noticeable symptoms at all. Untreated cervicitis can climb higher into the reproductive tract and become pelvic inflammatory disease.
How Mgen spreads
Mycoplasma genitalium is sexually transmitted, passed through genital contact during vaginal and other sexual activity. Because it's so often silent, transmission usually happens without either person knowing, and partners get caught in a reinfection loop if only one is treated.
How Mgen is tested
Diagnosis uses an FDA-cleared NAAT (nucleic acid amplification test) — the same molecular technology that detects chlamydia and gonorrhea — run on a urine sample or a urethral, penile-meatal, endocervical, or vaginal swab. Many of these swabs can be self-collected, and women's vaginal swabs and men's first-catch urine perform well.
Mgen is not on every standard STI panel, so you may have to specifically request the test or be tested because of a relevant syndrome. Current guidance is targeted, not universal: test men with recurrent non-gonococcal urethritis and women with recurrent cervicitis. No routine screening of asymptomatic people is recommended NYSDOH/Johns Hopkins. Macrolide-resistance testing should guide therapy, but it isn't commercially available in the US yet, so clinicians often treat as though resistance is likely.
In practice, testing is straightforward: a urine cup, a quick self-collected swab, or a brief exam, with results usually back in a few days. It's free or low-cost at health departments, Planned Parenthood, and Title X clinics. If you've had a possible exposure, timing matters — read up on when to test after exposure before you book, and you can get tested or compare testing providers to find an option that fits.
How Mgen is treated
Treatment is resistance-guided and uses two steps, always starting with doxycycline. The first phase lowers the bacterial load, and the second clears the infection, with the specific second drug chosen based on whether the strain resists macrolides.
| Scenario | Step 1 | Step 2 |
|---|---|---|
| Macrolide-sensitive strain | Doxycycline 100 mg twice daily for 7 days | Azithromycin 1 g once, then 500 mg daily for 3 days |
| Macrolide-resistant, or resistance testing unavailable | Doxycycline 100 mg twice daily for 7 days | Moxifloxacin 400 mg daily for 7 days |
The two steps exist because Mgen has quietly become a resistance problem. Macrolide-resistance mutations now exceed 50% in many areas — over 62% in one US STI clinic — so the old single dose of azithromycin frequently fails. Since the resistance test isn't sold here, many clinicians default to the moxifloxacin pathway or interpret based on response. Whatever regimen you're given, finish every pill even after you feel better, and ask whether your partner needs treating so you don't pass it back and forth. For the full breakdown of regimens and follow-up testing, see our guide to mgen treatment.
Complications if Mgen goes untreated
Left untreated, Mgen can move beyond nuisance symptoms. In women, the infection can drive ongoing cervicitis and ascend to cause pelvic inflammatory disease (PID) — infection and inflammation of the uterus, fallopian tubes, and surrounding tissue that can scar the tubes. That scarring is linked to preterm delivery and to infertility, with roughly a twofold increased risk.
In men, the main consequence is persistent or recurrent urethritis, inflammation that keeps returning and can be hard to resolve, particularly if it was misidentified as chlamydia and treated with the wrong antibiotic. A deeper look at long-term outcomes is in our overview of mgen complications.
How to prevent Mgen
Condoms used consistently lower the risk of sexually transmitted infections, Mgen included. Because the infection is so often symptomless, awareness and testing when you have a relevant syndrome catch what you'd otherwise miss.
- Use condoms every time to reduce transmission during genital contact.
- If you're diagnosed, your partners can be tested and treated if positive, which is the single best way to break the reinfection cycle.
- Don't stop treatment early; an incomplete course is one of the ways resistance gets a foothold.
- If urethritis or cervicitis keeps returning after treatment, ask specifically about an Mgen test rather than repeating the same regimen.
When to see a clinician
See a clinician if you have penile discharge or burning with urination, abnormal vaginal discharge or bleeding, pelvic pain, or symptoms that came back after you were treated for chlamydia or another STI. A recurrence is a strong cue to test for this specific organism. This diagnosis is common and treatable, and clinics handle it daily.