If you're allergic to penicillin and have syphilis, you can usually be treated with doxycycline (100 mg orally twice daily for 14 days for early disease, or 28 days for late latent) instead. The major exception is pregnancy: there's no safe alternative, so an allergic pregnant patient is desensitized to penicillin and treated with it anyway.
~3 wks; 10–90 days
by stage
| Item | Value |
|---|---|
| Primary sore | painless — ~3 wks; 10–90 days |
| Testing | 2 blood tests |
| Treatment | penicillin — by stage |
| Symptoms fade | ≠ cured |
How syphilis is treated when you're allergic to penicillin
Syphilis is caused by the bacterium Treponema pallidum, and it's curable with the right antibiotics CDC, About Syphilis. Penicillin G, given as an injection, is the preferred drug for every single stage, and no oral pill reliably replaces it CDC, STI Tx Guidelines. For a non-pregnant adult with a documented penicillin allergy, that means switching to a tetracycline-class antibiotic, and the duration depends on how long you've had the infection.
For primary, secondary, or early latent syphilis (acquired within the preceding year), the standard alternative is doxycycline 100 mg by mouth twice daily for 14 days, or tetracycline 500 mg by mouth four times daily for 14 days CDC, Primary & Secondary Syphilis. For late latent or unknown-duration disease, the doxycycline course stretches to 28 days. The shorter 14-day course is not sufficient at that stage, because the bacteria divide slowly and need a longer window of exposure to clear CDC, Latent Syphilis.
Azithromycin is not recommended for syphilis because of documented resistance. It may seem convenient, but it can fail. The early/late split that sets the schedule matters here. "Early latent" means acquired within the last 12 months, shown by a documented seroconversion, a fourfold-or-greater titer rise, or recent symptoms or exposure. Anything longer or undatable is treated as late latent.
If the infection has reached the brain, eyes, or ears — neurosyphilis, ocular syphilis, or otosyphilis — oral pills don't penetrate well enough, and treatment requires intravenous penicillin in the hospital. There is no comparable oral fallback for these forms, so confirm a penicillin allergy rather than assume it before you ever need it.
| Stage | First-line (penicillin) | If penicillin-allergic (non-pregnant) |
|---|---|---|
| Primary, secondary, early latent | Benzathine penicillin G 2.4 million units IM, single dose | Doxycycline 100 mg orally twice daily, 14 days |
| Late latent / unknown duration | Benzathine penicillin G 7.2 million units total (3 weekly IM doses) | Doxycycline 100 mg orally twice daily, 28 days |
| Neuro / ocular / otosyphilis | Aqueous crystalline penicillin G IV, 10–14 days | No reliable oral alternative — desensitize and treat with penicillin |
| Pregnancy (any stage) | Stage-appropriate penicillin | Penicillin desensitization, then penicillin |
What treatment is actually like
If you're on doxycycline, expect a course of pills taken twice a day. It's straightforward, but you have to finish every dose for it to work. Doxycycline can upset your stomach and make your skin more sensitive to sunlight, so take it with food and use sunscreen. Stopping early because you "feel fine" is one of the most common ways treatment fails.
Whether you get pills or an injection, many people develop a brief flu-like reaction within the first 24 hours: the Jarisch-Herxheimer reaction. It's fever, chills, headache, and muscle aches caused by large numbers of dying bacteria releasing inflammatory signals, not an allergy to the drug StatPearls, Jarisch-Herxheimer. It's most common when treating early syphilis and settles on its own within about a day. Don't mistake it for a penicillin allergy and don't stop treatment — over-the-counter fever reducers and fluids are usually all that's needed.
Most penicillin allergy is mislabeled. Over 90% of people who believe they're allergic are not, often because a childhood rash or a Herxheimer-type reaction was misattributed StatPearls, Syphilis. A formal allergy evaluation can open the door back to the most reliable cure.
Don't forget partner treatment
Curing yourself doesn't matter much if an untreated partner reinfects you. Recent sex partners should be notified, tested, and treated. Because a test can be falsely negative early after exposure, a partner with known recent contact is often treated presumptively rather than waiting. If you're unsure how soon a test turns positive, see when to test after exposure. Avoid sex until you and your partners have completed treatment and your clinician confirms you're clear.
Follow-up, retesting, and test-of-cure
Cure is confirmed by blood titers, not by how you feel. After treatment, you'll repeat a quantitative nontreponemal test (RPR or VDRL) on a schedule: at 6 and 12 months for primary or secondary syphilis, and at 6, 12, and 24 months for latent syphilis. A fourfold or greater drop in the titer — say, from 1:32 down to 1:8 — confirms an adequate response.
A titer that climbs fourfold instead of falling signals either reinfection or treatment failure. Sorting out which one matters, because they're managed differently — read more on syphilis reinfection vs treatment failure. You can catch it again, since treatment doesn't make you immune; here's can you get syphilis again after treatment?.
What happens if syphilis goes untreated
Syphilis can feel like it's getting better while it isn't. The first sore is usually painless and often hidden, and it heals on its own even as the bacteria move inward. Left untreated, it can progress to tertiary disease: cardiovascular syphilis (damage to the aorta and heart valves) and gummas (soft, destructive growths that can erode bone, skin, and organs).
Neurosyphilis (infection of the brain and spinal cord) and ocular syphilis (infection of the eye that can cause permanent vision loss) can occur at any stage; they're not just late complications StatPearls, Neurosyphilis. New vision changes, hearing loss, severe headache, or neurological symptoms in someone with syphilis are a medical emergency. Untreated syphilis also roughly doubles the risk of acquiring HIV, because the genital sores create an easy entry point.
In pregnancy the stakes are highest. Untreated syphilis harms the baby in 50 to 80% of cases — stillbirth, prematurity, newborn death, or congenital infection that can cause lifelong disability WHO, Syphilis. A penicillin-allergic pregnant patient is desensitized to penicillin and treated with it rather than given a weaker oral drug, because penicillin is the only treatment proven to protect the fetus. Congenital syphilis nearly doubled in four years, from 2,163 cases in 2020 to 3,882 in 2023, a 77% rise to its highest level in decades CDC, AtlasPlus 2023.
Preventing syphilis going forward
Once you're cured, prevention is about not getting reinfected. Correct and consistent condom use, mutual monogamy with a partner who's tested negative, and routine screening if you're in an at-risk group are the foundation. Syphilis is climbing fast — roughly 53,000 primary-and-secondary cases were reported in 2023, with sharp regional concentration — so regular testing matters even if you feel fine. Knowing what early signs to watch for helps too; see syphilis symptoms in men vs women.
There's also a newer biomedical option for some people. DoxyPEP — taking 200 mg of doxycycline within 72 hours after sex — reduced syphilis acquisition by about 73% in studies, and the CDC recommends it through shared decision-making for gay and bisexual men and transgender women who've had a bacterial STI in the past 12 months CDC, DoxyPEP 2024. It's not for everyone, so discuss it with a clinician.
When to see a clinician
See a clinician promptly if you've had a known exposure, notice a painless sore or an unexplained rash (especially on the palms or soles), or your follow-up titer isn't dropping. Go urgently for any new vision change, hearing loss, or neurological symptom. Diagnosis is a simple blood draw, but it takes two tests, a screening test plus a confirmatory one, and it can read negative very early after exposure CDC, Syphilis Lab 2024. You can get tested easily, and if you're weighing options you can compare testing providers.