Antibiotics can cure the most common bacterial sexually transmitted infections, including chlamydia, gonorrhea, and syphilis, but they are powerless against viral STDs like HIV, herpes, hepatitis B, and HPV. That distinction between bacterial and viral is the single most important thing to understand about STD treatment, because it determines whether a cure is possible at all. The picture gets more complicated once you factor in rising drug resistance, the specific antibiotic matched to each infection, and a few STDs that fall outside the bacterial-versus-viral binary entirely.
The Antibiotics That Treat Chlamydia
Chlamydia is the most frequently reported bacterial STD, and for years, treatment looked simple: a single dose of azithromycin or a seven-day course of doxycycline. Both worked well enough that clinicians often treated them as interchangeable. That view has shifted. A randomized trial found zero treatment failures with doxycycline, while azithromycin failed in about 3% of urogenital chlamydia cases.1PubMed Central. Azithromycin versus Doxycycline for Urogenital Chlamydia trachomatis Infection That gap widens dramatically for rectal chlamydia infections, where azithromycin cured only about 79% of cases compared to roughly 96% with doxycycline.2PubMed Central. Treatment Effectiveness of Azithromycin and Doxycycline in Uncomplicated Rectal and Vaginal Chlamydia trachomatis Infections in Women: A Multicenter Observational Study (FemCure)
The practical result is that doxycycline (100 mg taken twice daily for a week) has become the preferred first-line treatment for chlamydia. Azithromycin in a single dose still works for most vaginal infections, and it remains a reasonable alternative when someone genuinely cannot complete a week-long regimen. But if the infection is rectal, or if treatment needs to be as reliable as possible, doxycycline is the stronger bet. The trade-off is obvious: remembering to take pills for seven days versus a single dose you swallow in the clinic. That convenience factor is real, and it is part of why azithromycin was so popular for so long, but cure rates matter more.
Gonorrhea and the Shrinking List of Options
Gonorrhea has burned through nearly every antibiotic class thrown at it over the past century. Sulfonamides, penicillins, tetracyclines, fluoroquinolones: one by one, the bacterium Neisseria gonorrhoeae developed resistance to each. Today, ceftriaxone given as an intramuscular injection is essentially the only reliable first-line option left.3PubMed Central. The Management of Gonorrhea in the Era of Emerging Antimicrobial Resistance: What Primary Care Clinicians Should Know The CDC raised the recommended dose to 500 mg (or 1 g for people over 150 kg) in its 2021 treatment guidelines, reflecting a desire to stay ahead of creeping resistance.
The resistance problem is not theoretical. Strains with high-level resistance to ceftriaxone have already been identified, and researchers have warned that gonorrhea could become untreatable in certain situations.4PubMed Central. Antibiotic resistance in Neisseria gonorrhoeae: origin, evolution, and lessons learned for the future The bacterium is unusually good at acquiring resistance, both through mutations that change the shape of the proteins antibiotics target and through pumping drugs back out of the cell before they can do their work.5Infectious Microbes and Diseases. Antibiotic Resistance and Treatment Options for Multidrug-Resistant Gonorrhea Few new anti-gonococcal drugs are in the pipeline, which makes the situation genuinely precarious.
If chlamydia has not been ruled out, which it often hasn’t at the time of treatment, clinicians typically add doxycycline for a week to cover both infections simultaneously.6PubMed Central. Update to CDC’s Treatment Guidelines for Gonococcal Infection, 2020 You cannot take a pill at home for gonorrhea the way you can for chlamydia; the injection is necessary because oral antibiotics are no longer reliable enough against this particular bacterium.
Syphilis and the Staying Power of Penicillin
Syphilis is one of the rare success stories in infectious disease where the original treatment, penicillin, still works beautifully more than 70 years after it was introduced. A single intramuscular injection of benzathine penicillin G clears the syphilis-causing spirochete Treponema pallidum from the blood and ulcer sites remarkably fast. In a study tracking bacterial DNA after treatment, T. pallidum was undetectable in any sample collected more than 56 hours after the shot, with a clearance half-life in the blood of under six hours.7PubMed Central. Rapid Treponema pallidum clearance from blood and ulcer samples following single dose benzathine penicillin treatment of early syphilis
For early syphilis (primary, secondary, or early latent), that single injection is the standard cure. Late latent syphilis or syphilis of unknown duration requires three injections spaced a week apart. There is no well-proven alternative for pregnant people with syphilis: penicillin is the only treatment known to reliably prevent transmission to the fetus. When a pregnant person reports a penicillin allergy, the standard approach is desensitization, gradually introducing the drug under close medical supervision, followed by the full treatment dose.8Journal of Case Reports and Images in Obstetrics and Gynecology. Successful penicillin desensitization in a pregnant woman with latent syphilis and penicillin allergy For non-pregnant adults with a penicillin allergy, doxycycline is a backup option, but the evidence supporting it is much thinner than for penicillin.
Mycoplasma Genitalium, the Newer Concern
Mycoplasma genitalium is a bacterial STI that has been getting more clinical attention in recent years. It can cause urethritis and cervicitis, and standard-issue treatment does not always work because the organism has high rates of resistance to macrolide antibiotics like azithromycin. A resistance-guided approach, where clinicians first use doxycycline and then choose a follow-up drug based on resistance testing, has shown cure rates of 92% or higher.9PubMed Central. Outcomes of Resistance-guided Sequential Treatment of Mycoplasma genitalium Infections: A Prospective Evaluation The trouble is that this kind of testing is not yet routine everywhere, and without it, clinicians are sometimes guessing at which drugs the infection will respond to.
This is a pattern worth watching. M. genitalium is not yet reported as commonly as chlamydia or gonorrhea in most public health surveillance systems, but it is increasingly recognized as a cause of persistent or recurrent symptoms that do not clear with standard chlamydia treatment. If you have been treated for chlamydia and your symptoms linger, M. genitalium testing is worth discussing with your provider.
Trichomoniasis Is Curable but Not With a Typical Antibiotic
Trichomoniasis is one of the most common curable STDs worldwide, but the organism that causes it, Trichomonas vaginalis, is not a bacterium. It is a protozoan parasite. The standard treatment is metronidazole, a drug derived from the antibiotic azomycin but classified as an antiparasitic.10PubMed Central. Treatment of infections caused by metronidazole-resistant Trichomonas vaginalis A single dose has been the traditional first-line approach,11PubMed Central. Single-dose versus 7-day-dose metronidazole for the treatment of trichomoniasis in women: an open-label, randomised controlled trial though multi-day courses are sometimes used, particularly for recurrent or stubborn infections.
The reason this matters for the “what antibiotics treat STDs” question is that people commonly lump trichomoniasis in with infections like chlamydia and gonorrhea, assuming a standard antibiotic like azithromycin or doxycycline will handle it. Those drugs do nothing for trich. Metronidazole (or the related drug tinidazole) is required. Metronidazole-resistant trich does exist but is uncommon; when it occurs, higher doses or tinidazole are typically tried.
What Antibiotics Cannot Cure
The viral STDs are where antibiotics hit a wall. No antibiotic of any kind can cure HIV, herpes (HSV-1 or HSV-2), human papillomavirus (HPV), or hepatitis B. These viruses integrate into your cells or persist in ways that antibacterial drugs simply cannot reach. Antiviral medications exist for all four, but the word “treat” here means managing symptoms, reducing transmission risk, or suppressing viral replication. It does not mean eliminating the infection.
Herpes, for example, can be managed with daily suppressive antiviral therapy (drugs like valacyclovir), which reduces outbreaks and cuts the risk of passing the virus to a partner. But stop the medication and the virus, which hides in nerve cells, can reactivate. HIV is similar in principle: antiretroviral therapy keeps viral levels undetectable and prevents transmission, but the virus persists in reservoirs the immune system cannot clear. HPV often clears on its own through the immune response, but when it does not, there is no drug that eliminates it; treatment addresses the consequences (warts, precancerous cells) rather than the virus itself. Hepatitis B has antiviral options for chronic infections but, again, a true cure is elusive for most patients.
The common misconception here is that any STD can be cured if you just get the right prescription. People sometimes delay testing because they assume that whatever it is, a pill will fix it. For the bacterial infections, that is largely true. For the viral ones, early detection still matters enormously for health outcomes and for preventing transmission, but the outcome is management, not cure.
Chlamydia Treatment During Pregnancy
Pregnancy restricts the antibiotic toolbox. Doxycycline, the top choice for chlamydia in the general population, is contraindicated during pregnancy because of potential harm to fetal bone and tooth development. The alternatives are erythromycin, amoxicillin, clindamycin, and single-dose azithromycin, with cure rates of 86%, 92%, 93%, and 95% respectively.12PubMed. Treatment of Chlamydia trachomatis infections in pregnant women Azithromycin’s high cure rate and single-dose convenience make it particularly appealing in this setting, which is ironic given the trend away from azithromycin for chlamydia in non-pregnant adults.
Untreated chlamydia in pregnancy raises the risk of preterm birth, low birth weight, and neonatal eye and lung infections. This is one situation where the limits of the safer drugs genuinely matter: a cure rate of 86% (erythromycin) means about one in seven treated pregnancies may still harbor the infection. Test-of-cure after treatment is more important here than in the general population.
The Difficulty of Confirming Cure
After antibiotic treatment, many people reasonably wonder how they can be sure the infection is gone. The standard recommendation for chlamydia and gonorrhea is a test-of-cure about three to four weeks after treatment, but even that approach has wrinkles. Highly sensitive molecular tests (NAATs) can detect fragments of dead bacteria for weeks after a successful cure, producing a positive result that does not actually mean the infection persists. A single positive test result taken three to eight weeks after treatment cannot reliably distinguish between treatment failure, reinfection, and leftover bacterial DNA.13PubMed Central. Chlamydia trachomatis Test-of-Cure Cannot Be Based on a Single Highly Sensitive Laboratory Test Taken at Least 3 Weeks after Treatment
The practical advice: if your post-treatment test comes back positive, it does not automatically mean the antibiotic failed. Your clinician will likely consider whether reinfection from an untreated partner is the more likely explanation, especially since reinfection is far more common than true drug resistance with chlamydia. Testing too early (within a week or two of treatment) is especially unreliable and can cause unnecessary worry.
Making Sure Partners Get Treated
Curing your own infection accomplishes little if your sexual partner is still carrying the same bacterium. Reinfection from an untreated partner is one of the most common reasons people end up back in the clinic weeks after successful treatment. Expedited partner therapy, where your clinician gives you medication or a prescription to pass along to your partner without requiring them to come in for their own visit, was designed to close this gap.14PubMed. The Impact of Prescriptions on Sex Partner Treatment Using Expedited Partner Therapy for Chlamydia trachomatis Infection, New York City, 2014-2015
The concept is sensible, but the real-world results are mixed. A study in adolescent females found that reinfection rates for chlamydia and trichomoniasis were not significantly different before and after expedited partner therapy was introduced, though the trend was in the right direction for those who actually accepted the medication.15PubMed. Expedited Partner Therapy in Female Adolescents: A Study of Acceptance and the Impact on Reinfection Rates The problem is not that the drugs do not work when partners take them. The problem is that many partners never fill the prescription, never take the pills, or were not the only partner. Still, in situations where a partner genuinely cannot or will not see a clinician, handing them a course of antibiotics is better than hoping they will make their own appointment.
Doxycycline as Post-Exposure Prevention
One of the more interesting recent developments is using doxycycline after sexual exposure to prevent bacterial STDs from taking hold in the first place, a strategy called doxy-PEP (post-exposure prophylaxis). The idea is straightforward: take 200 mg of doxycycline within 72 hours of condomless sex, and the drug kills bacteria before an infection establishes itself.
A landmark trial found that doxycycline taken this way reduced the incidence of any bacterial STI by about two-thirds compared to standard care, with particularly dramatic reductions in chlamydia and syphilis.16PubMed Central. Postexposure Doxycycline to Prevent Bacterial Sexually Transmitted Infections A systematic review and meta-analysis across multiple studies confirmed a roughly 65% reduction in chlamydia risk and a 77% reduction in syphilis risk.17PubMed. Efficacy of postexposure prophylaxis with doxycycline (Doxy-PEP) in reducing sexually transmitted infections: a systematic review and meta-analysis
The catch is gonorrhea. In controlled trials, doxy-PEP showed some reduction in gonorrhea, but the meta-analysis found the effect was not statistically significant overall. Real-world observational data painted an even more troubling picture: doxy-PEP use was actually associated with higher odds of gonorrhea infection, not lower.18PubMed. Real-World Clinical Outcomes of Doxycycline Post-Exposure Prophylaxis for Bacterial STI Prevention This likely reflects the fact that gonorrhea already has widespread tetracycline resistance. Exposing the gonococcus to sub-therapeutic doxycycline concentrations could even be selecting for more resistant strains, though that concern is still being studied.
Doxy-PEP is currently recommended primarily for men who have sex with men and transgender women who are at high risk for bacterial STIs. It is not a blanket recommendation for all sexually active people, in part because of the uncertain gonorrhea picture and in part because widespread doxycycline use could accelerate resistance in multiple organisms, not just N. gonorrhoeae. If you are considering it, the conversation with your provider should include whether gonorrhea is a significant part of your risk profile, because doxy-PEP does not meaningfully address that one.
Why Antibiotic Resistance in STDs Should Concern Everyone
The gonorrhea resistance story is the most dramatic, but it is not isolated. M. genitalium has high rates of macrolide resistance. Metronidazole-resistant trich, while uncommon, exists. And the worry with doxy-PEP is that widespread prophylactic use of doxycycline could erode the effectiveness of that drug for chlamydia treatment, which would be a significant setback given that doxycycline recently became the preferred first-line option.
The resistance mechanisms in gonorrhea are well-characterized: the bacterium mutates the targets that antibiotics bind to, so the drug can no longer latch on, and it ramps up molecular pumps that eject drugs from the cell before they reach effective concentrations.5Infectious Microbes and Diseases. Antibiotic Resistance and Treatment Options for Multidrug-Resistant Gonorrhea Gonorrhea is also unusually good at picking up resistance genes from other bacteria in the environment. Each class of antibiotics we deploy against it has a limited lifespan of effectiveness, and with few new drugs in development, the gap between the last working antibiotic and the next one is genuinely concerning.4PubMed Central. Antibiotic resistance in Neisseria gonorrhoeae: origin, evolution, and lessons learned for the future
For you as a patient, the practical takeaway is that completing prescribed courses exactly as directed, getting proper testing rather than self-treating, and making sure partners are treated are not just personal health measures. They are part of slowing the resistance trend for everyone. Skipping doses, sharing leftover antibiotics, or taking the wrong drug for the wrong infection all create conditions where partially suppressed bacteria survive and adapt.
When the First Treatment Does Not Work
Treatment failure can happen for several reasons, and true drug resistance is only one of them. More commonly, what looks like failure is actually reinfection from an untreated partner, incomplete adherence to a multi-day regimen, or a misdiagnosis where the wrong pathogen was targeted. If you test positive again after treatment, your clinician will typically consider all of these possibilities before assuming the bacterium itself was resistant.
For gonorrhea, if the standard ceftriaxone injection does not work, the options become limited. Higher doses, alternative injectable cephalosporins, or combination regimens may be tried, but these are off-guideline territory that infectious disease specialists handle on a case-by-case basis. For chlamydia, switching from azithromycin to doxycycline (or vice versa) usually resolves the problem if the first drug was the issue. For syphilis, true penicillin resistance has essentially never been documented; apparent failures are more often due to late-stage disease requiring longer treatment or to reinfection.
M. genitalium is the exception where genuine resistance is the most likely explanation for treatment failure. Resistance-guided testing, where the lab checks which drug classes the organism is still susceptible to before prescribing, is the approach that gets cure rates above 92%.9PubMed Central. Outcomes of Resistance-guided Sequential Treatment of Mycoplasma genitalium Infections: A Prospective Evaluation Without that testing, clinicians are often cycling through drugs empirically, which can take multiple rounds and contributes to further resistance.