What STDs Does Azithromycin Treat vs. Not Treat?

Azithromycin reliably treats a handful of bacterial STDs, performs poorly against several others, and does nothing at all for viral or parasitic infections. For years it was the go-to single-dose cure for chlamydia, and it still works well for chancroid and granuloma inguinale. But the landscape has shifted: growing evidence of lower cure rates for rectal chlamydia, widespread resistance in Mycoplasma genitalium, and near-total loss of usefulness against gonorrhea and syphilis have narrowed where azithromycin fits in STD treatment. The picture is more complicated than “take the Z-pack and you’re done.”

Urogenital Chlamydia

For decades, a single 1-gram oral dose of azithromycin was the default treatment for uncomplicated genital chlamydia infections. The appeal was obvious: one dose, taken in front of a clinician, and you were finished. No week of pills to forget. That convenience kept azithromycin at the top of treatment guidelines for a long time, and it genuinely works for most vaginal and urethral chlamydia infections. In vaginal infections, cure rates for azithromycin and its main competitor, doxycycline, are similar, both landing above 93%.¹

However, a head-to-head trial found that while doxycycline had zero treatment failures, azithromycin failed in about 3% of urogenital chlamydia cases. That may sound small, but it was enough that azithromycin could not be proven equivalent to doxycycline by the study’s own standards.² This finding, combined with evidence from other body sites (more on that below), led the CDC in 2021 to change its primary recommendation: doxycycline, taken twice daily for seven days, is now the preferred treatment for chlamydia. Azithromycin remains an alternative, but it is no longer the first choice.

Rectal Chlamydia, Where the Gap Widens

The most dramatic difference between azithromycin and doxycycline shows up in rectal chlamydia infections, which are common among men who have sex with men and also occur in women. In a large trial, doxycycline cured about 97% of asymptomatic rectal chlamydia cases, while azithromycin cured only about 76%, a gap of roughly 20 percentage points.³ An observational study of women with rectal chlamydia found a similar pattern, with azithromycin curing about 79% compared to doxycycline’s 96%.¹

The reason is partly pharmacological. Azithromycin concentrates well in many tissues, but rectal tissue may not accumulate enough of the drug for long enough to kill chlamydia reliably. Doxycycline, taken over a full week, maintains a longer window of antimicrobial exposure. If you have been diagnosed with rectal chlamydia, azithromycin is not the right choice.

Lymphogranuloma Venereum

Lymphogranuloma venereum (LGV) is caused by specific aggressive strains of Chlamydia trachomatis and requires longer treatment than ordinary chlamydia. A standard single dose of azithromycin is not enough. A case series found that patients who received the typical 1-gram azithromycin contact treatment for LGV went on to develop established infections anyway, highlighting that the single dose is insufficient for this particular form of chlamydial disease.⁴ LGV instead requires 21 days of doxycycline. The problem for clinicians is that LGV can look clinically identical to ordinary chlamydia in the early stages, and some patients get the wrong regimen before the invasive strain is identified.

Gonorrhea

Azithromycin was never a standalone cure for gonorrhea, but for several years it was paired with an injectable cephalosporin antibiotic (ceftriaxone) in a dual-therapy approach. The idea was that two drugs hitting Neisseria gonorrhoeae through different mechanisms would slow the development of resistance. That era is effectively over. Reports of gonorrhea strains with high-level azithromycin resistance emerged in multiple countries, and current guidelines now recommend ceftriaxone alone as first-line therapy, dropping azithromycin from the combination entirely.

The gut-related side effects of azithromycin were also a practical barrier when it was used at the higher 2-gram dose sometimes employed against gonorrhea. At that dose, roughly half of patients experienced diarrhea and over 40% had nausea, compared to about a quarter and 14% respectively at the standard 1-gram dose.⁵ With resistance rising and tolerability poor at the higher dose, there is little reason to reach for azithromycin in gonorrhea treatment today.

Syphilis

Azithromycin does not reliably treat syphilis and should not be used for it. Treponema pallidum, the bacterium that causes syphilis, has developed point mutations that confer resistance to macrolide antibiotics including azithromycin. A study of treatment failures in Shanghai found that all patients who failed azithromycin therapy for primary and secondary syphilis carried a specific resistance mutation in the bacterium’s genetic material.⁶ These resistance-conferring mutations have been detected widely in T. pallidum populations across several continents. Penicillin G (given by injection) remains the gold-standard syphilis treatment, and for penicillin-allergic patients, doxycycline is the usual alternative. Azithromycin is essentially off the table here.

Mycoplasma Genitalium

This is one of the more frustrating chapters in azithromycin’s story. Mycoplasma genitalium causes urethritis in men and cervicitis and pelvic inflammatory disease in women, and azithromycin was initially effective against it. But M. genitalium has a remarkable ability to develop resistance to macrolide antibiotics, and it can do so quickly, sometimes within a single course of treatment.

Researchers have documented that when azithromycin fails to clear M. genitalium, the surviving bacteria frequently carry newly acquired mutations in the 23S ribosomal RNA gene, the same stretch of genetic code that azithromycin targets.⁷ In other words, a single standard dose of azithromycin can select for resistant mutants that were present at low levels before treatment, leaving the patient with an infection that is now harder to treat.⁸ Reviews of the evidence have confirmed a strong link between these 23S rRNA mutations and azithromycin treatment failure.⁹

The prevalence of resistance is staggering in some settings. In a study of clinical specimens from Los Angeles, 80% of M. genitalium-positive samples carried at least one mutation associated with azithromycin resistance.¹⁰ Because of this, many guidelines now recommend testing for macrolide resistance before prescribing azithromycin for M. genitalium. If resistance mutations are found, a fluoroquinolone (moxifloxacin) is used instead. Treating M. genitalium blindly with azithromycin risks not only failing to cure the patient but breeding a more dangerous version of the infection.

Non-Gonococcal Urethritis

Non-gonococcal urethritis (NGU) is a catch-all diagnosis for urethral inflammation not caused by gonorrhea. Chlamydia and M. genitalium are the two most common identified culprits, but in many cases no specific pathogen is found. Azithromycin and doxycycline perform similarly overall for NGU, with no significant difference in clinical cure rates in a randomized trial. Both regimens cured about 69–75% of patients at the first follow-up and roughly 44–49% by the final visit, the drop reflecting recurrence and the difficulty of clearing M. genitalium in particular.¹¹

The practical issue is that if your NGU turns out to be caused by M. genitalium, treating it with azithromycin can induce resistance, as described above. If it is caused by chlamydia, azithromycin works but doxycycline works a bit better. The overall trend in clinical practice has been to lean toward doxycycline first, then deal with M. genitalium specifically if symptoms persist.

Chancroid and Granuloma Inguinale

These two ulcerative STDs are uncommon in high-income countries but still occur in parts of the tropics and subtropics, and azithromycin is genuinely useful for both. In a randomized trial comparing a single 1-gram azithromycin dose to a multi-day course of erythromycin for chancroid (caused by Haemophilus ducreyi), cure rates were about 89% with azithromycin, essentially equivalent to the comparator, with the huge advantage of single-dose simplicity.¹²

Granuloma inguinale (donovanosis), caused by Klebsiella granulomatis, also responds to azithromycin. A pilot study found that all patients treated with azithromycin (using either a 1-gram weekly or a 500-mg daily regimen over several weeks) had their genital ulcers heal completely, with no relapses over follow-up periods extending to seven months.¹³ For both of these infections, azithromycin remains a recommended option, particularly in resource-limited settings where ensuring multi-day adherence is difficult.

Viral and Parasitic STDs

Azithromycin is an antibiotic. It has no activity against viruses or parasites. That means it does nothing for herpes (HSV-1 or HSV-2), human papillomavirus (HPV), HIV, hepatitis B or C, or molluscum contagiosum. It also does not treat trichomoniasis, which is caused by a protozoan parasite (Trichomonas vaginalis) and requires metronidazole or tinidazole. This seems obvious, but in practice, patients prescribed azithromycin for one STD sometimes assume they have been treated for everything detected or suspected. They have not.

Azithromycin in Pregnancy

Pregnancy is one setting where azithromycin still has a clear role for chlamydia. Doxycycline is contraindicated during pregnancy because of risks to fetal bone and tooth development. A meta-analysis of randomized trials found that azithromycin is as effective as erythromycin for clearing chlamydia in pregnant women but causes fewer side effects, making it the preferred option in this population.¹⁴ This is worth knowing because the shift toward doxycycline as first-line chlamydia treatment does not apply to pregnant patients, for whom azithromycin remains front and center.

Pelvic Inflammatory Disease

Pelvic inflammatory disease (PID) is an ascending infection of the uterus and fallopian tubes, usually caused by chlamydia, gonorrhea, or a mix of bacteria. Treatment always involves combination antibiotic regimens, not a single drug. Azithromycin-based regimens (typically 1 gram weekly for two weeks, combined with other antibiotics) have been studied in this context. One trial found that azithromycin-based treatment achieved 100% clearance of chlamydia versus 92% for doxycycline-based treatment, with similar performance against other organisms when used in combination.¹⁵ PID treatment is complex and usually involves an injectable antibiotic alongside the oral one, so azithromycin here is always part of a larger regimen.

Why Azithromycin Concentrates So Well (and Why It Still Fails)

Part of what makes azithromycin unusual among antibiotics is how it distributes in the body. It accumulates inside immune cells called phagocytes, which then carry it directly to sites of infection. It reaches tissue concentrations far higher than what shows up in the blood, and it clears slowly, meaning a single dose can keep working for days.¹⁶ This property is what made a single-dose cure for chlamydia possible in the first place. But high tissue concentration does not help when bacteria carry genetic mutations that make them fundamentally insensitive to the drug. The resistance story, whether in syphilis, gonorrhea, or M. genitalium, is about the target being altered so that even high drug levels at the infection site cannot kill the organism.

Expedited Partner Therapy

One area where azithromycin’s single-dose convenience still matters is expedited partner therapy (EPT). EPT means giving medication directly to a patient so they can hand it to their sexual partner, bypassing the need for the partner to visit a clinic. This approach works best with a single-dose drug that a partner can take immediately, and azithromycin fits that profile perfectly. Over 30 U.S. states now permit EPT for chlamydia, and evaluations of EPT programs generally show higher rates of partner treatment, though not all studies find reduced reinfection rates.¹⁷ Even as doxycycline has taken over as the preferred chlamydia treatment for the patient sitting in the clinic, azithromycin often remains the practical choice for the partner who may never come in.

Side Effects and Tolerability at Different Doses

At the standard 1-gram dose used for chlamydia, azithromycin is generally well tolerated. The most common complaints are gastrointestinal: nausea, diarrhea, and abdominal discomfort. These side effects become much more pronounced at higher doses. A comparison across dosing regimens found that nausea affected roughly 14% of people taking 1 gram but over 43% of those taking a single 2-gram dose. Diarrhea followed a similar pattern, hitting about a quarter of patients at 1 gram and half at 2 grams. Splitting the 2-gram dose into two 1-gram doses taken hours apart reduced nausea and diarrhea somewhat but did not eliminate the problem.⁵ Vomiting, which can undermine absorption and lead to treatment failure, was uncommon at the 1-gram dose (about 1%) but climbed to nearly 4% at the 2-gram single dose. These tolerability numbers mattered most when the 2-gram dose was used in gonorrhea dual therapy, a practice that has since been discontinued in most guidelines.

Rare but serious side effects include cardiac arrhythmias (prolongation of the QT interval on an electrocardiogram) and liver injury. These risks are low enough that they do not change prescribing for most patients, but people with existing heart-rhythm disorders or liver disease should mention that to their prescriber.

How Resistance Testing Is Changing the Picture

One of the most significant shifts in STD management is the move toward resistance-guided therapy, particularly for M. genitalium. Rather than prescribing azithromycin and hoping for the best, clinicians in many settings now test for macrolide resistance mutations before choosing an antibiotic. If the test shows the infection carries 23S rRNA mutations, azithromycin is skipped entirely in favor of moxifloxacin or other alternatives. This approach is still not universally available, as the molecular tests are relatively new and not offered at every clinic, but it represents the direction the field is moving. For chlamydia and gonorrhea, routine resistance testing before treatment is not yet standard practice, though laboratory surveillance programs track resistance patterns at the population level to guide public-health recommendations.

The broader lesson from the azithromycin story is that antibiotic convenience comes with a cost. The same properties that made azithromycin attractive for single-dose treatment, a long half-life and slow clearance, also create a prolonged window of sub-therapeutic drug levels in the body. That window is where resistant mutants can emerge and multiply, a dynamic that has been documented most vividly with M. genitalium but plays out across multiple pathogens. The era of reaching reflexively for azithromycin as a one-size-fits-all STD treatment has ended, replaced by a more targeted approach that matches the antibiotic to both the organism and its resistance profile.

1
PubMed Central. Treatment Effectiveness of Azithromycin and Doxycycline in Uncomplicated Rectal and Vaginal Chlamydia trachomatis Infections in Women: A Multicenter Observational Study (FemCure)
2
PubMed Central. Azithromycin versus Doxycycline for Urogenital Chlamydia trachomatis Infection
3
PubMed. Azithromycin or Doxycycline for Asymptomatic Rectal Chlamydia trachomatis
4
PubMed. Pitfalls in the diagnosis and management of inguinal lymphogranuloma venereum: important lessons from a case series
5
PubMed Central. Comparison of gastrointestinal side effects from different doses of azithromycin for the treatment of gonorrhoea
6
PubMed Central. Azithromycin Treatment Failure Among Primary and Secondary Syphilis Patients in Shanghai
7
PubMed. Azithromycin treatment failure in Mycoplasma genitalium-positive patients with nongonococcal urethritis is associated with induced macrolide resistance
8
PubMed. Selection of Mycoplasma genitalium strains harbouring macrolide resistance-associated 23S rRNA mutations by treatment with a single 1 g dose of azithromycin
9
PubMed Central. Mycoplasma genitalium infection: current treatment options, therapeutic failure, and resistance-associated mutations
10
PubMed Central. Prevalence of Mycoplasma genitalium and Azithromycin-Resistant Infections Among Remnant Clinical Specimens, Los Angeles
11
PubMed Central. Re-Evaluating the Treatment of Nongonococcal Urethritis: Emphasizing Emerging Pathogens–A Randomized Clinical Trial
12
PubMed. Single dose azithromycin for the treatment of chancroid: a randomized comparison with erythromycin
13
PubMed Central. Pilot study of azithromycin in the treatment of genital donovanosis
14
International Journal of Antimicrobial Agents. Single-dose azithromycin versus erythromycin or amoxicillin for Chlamydia trachomatis infection during pregnancy: a meta-analysis of randomised controlled trials
15
PubMed Central. Azithromycin for PID beats doxycycline on all counts
16
Pharmacology & Therapeutics. Azithromycin: Mechanisms of action and their relevance for clinical applications
17
PubMed Central. Expedited partner therapy for sexually transmitted infections