The Jarisch-Herxheimer reaction is a short-lived flare of fever, chills, headache, and muscle aches that appears within the first day after syphilis treatment, usually within a couple of hours of the first penicillin injection. It's caused by dying bacteria releasing inflammatory signals, not a drug allergy, and it settles on its own within about a day StatPearls.
~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 |
If you felt fine before your shot and then suddenly felt feverish and achy afterward, that's the reaction most people are searching for. It's expected, harmless in most patients, and it is NOT a reason to avoid penicillin in the future. Below is what's happening, why it's so common with early syphilis, and what to do.
What is the Jarisch-Herxheimer reaction?
Syphilis is caused by the bacterium Treponema pallidum CDC. When penicillin kills a large number of these organisms quickly, the dying bacteria spill out fragments and toxins that your immune system reacts to all at once. The result is a brief inflammatory surge: fever, shaking chills, pounding headache, sore muscles, and sometimes a temporary worsening of a rash or sore.
The reaction is most common when treating early-stage disease, when the bacterial load is highest. It affects roughly 95% of people treated for secondary syphilis and typically resolves within about 24 hours. This is dying bacteria, not a penicillin allergy. People who confuse the two end up refusing the one drug that reliably cures the infection.
How to tell it apart from a penicillin allergy
A true penicillin allergy tends to show up as hives, itching, swelling of the lips or throat, wheezing, or a drop in blood pressure, all signs of an immune reaction to the drug itself. The Jarisch-Herxheimer reaction looks like a brief flu: fever, chills, headache, aches, no airway swelling, and it fades within a day. If you ever develop true allergy symptoms after an injection, that's an emergency and needs immediate care. The achy fever alone is not.
How syphilis is treated — the regimen
Penicillin G, given by injection, is the preferred drug for every stage of syphilis CDC STI Guidelines. The dose and schedule depend entirely on the stage:
| Stage | Standard regimen |
|---|---|
| Primary, secondary, or early latent (acquired within the preceding 12 months) | Benzathine penicillin G 2.4 million units IM in a single dose |
| Late latent / unknown duration or tertiary | Benzathine penicillin G 7.2 million units total — three doses of 2.4 million units IM each, one week apart |
| Neuro-, ocular, or otosyphilis | Aqueous crystalline penicillin G 18–24 million units per day, as 3–4 million units IV every 4 hours (or continuous infusion) for 10–14 days |
| Penicillin allergy, non-pregnant (early stages) | Doxycycline 100 mg orally twice daily for 14 days |
| Penicillin allergy, non-pregnant (late latent) | Doxycycline 100 mg orally twice daily for 28 days |
The correct product for early syphilis is benzathine penicillin G (Bicillin L-A). The combination product Bicillin C-R, which mixes benzathine and procaine, is NOT an acceptable substitute and has led to treatment-failure errors CDC P&S Syphilis. If you're getting the injection, it's reasonable to confirm the product name with your clinician.
For neurosyphilis or ocular involvement, an alternative is procaine penicillin G 2.4 million units IM daily plus probenecid 500 mg orally four times daily, both for 10–14 days CDC Neurosyphilis. Azithromycin is not recommended for syphilis at all because of documented resistance. For full stage-by-stage dosing, see the syphilis symptoms overview, which lines up symptoms with the treatment schedule.
What treatment is actually like
For early syphilis, treatment is a single intramuscular injection, usually into the buttock. It's a thick suspension, so the shot can sting and the muscle may ache for a day or two. For late or unknown-duration disease, plan on three of these injections spaced a week apart; missing the interval can mean restarting the series, so put the dates on your calendar.
Within the first 24 hours, a flu-like reaction is possible: fever, chills, headache, muscle aches. It's the Jarisch-Herxheimer reaction, not an allergy. Most people manage it with rest, fluids, and an over-the-counter fever reducer, and it passes on its own. Pregnant patients should be told about it specifically, since fever and contractions can occur; they should still be treated, but watched.
Partner treatment
Syphilis is passed through direct contact with a sore or rash during sex, so anyone you've been intimate with recently needs evaluation and treatment, even if they feel completely well. The first sore is painless and easy to miss, so an exposed partner can be infectious without knowing it. Notifying partners, or asking your clinic to help anonymously, stops the chain and protects you from being reinfected by an untreated partner.
If you and your partner are unsure who tested when, our guide on syphilis reinfection vs treatment failure explains how clinicians use blood titers to tell a new infection from a treatment that didn't fully work.
Follow-up, retesting, and confirming cure
Feeling better, including the Jarisch-Herxheimer reaction fading, does not confirm cure. Cure is proven by blood titers over time CDC Latent Syphilis. For primary and secondary syphilis, you repeat quantitative nontreponemal titers (RPR or VDRL) at 6 and 12 months; for latent syphilis, at 6, 12, and 24 months.
A fourfold or greater drop in the titer, for example from 1:32 down to 1:8, confirms the treatment worked. A fourfold rise in the other direction signals reinfection or treatment failure and means you need to be re-evaluated. Because this is a blood test, follow-up is a simple draw; see how the numbers work on our syphilis test page, and you can get tested when your follow-up window comes due.
What happens if syphilis is left untreated
Untreated syphilis doesn't stay still. The early sores and rash heal on their own while the bacteria keep moving inward. Over years it can progress to tertiary syphilis, which damages the heart and large blood vessels (cardiovascular syphilis), forms destructive soft growths in tissue and bone (gummas), and injures multiple organs StatPearls.
Two complications can strike at any stage, not just late: neurosyphilis (infection of the brain and spinal cord, causing strokes, confusion, or paralysis) and ocular syphilis (infection of the eye that can cause permanent blindness) StatPearls Neurosyphilis. New vision changes or neurological symptoms in someone with syphilis are a medical emergency. Untreated syphilis also roughly doubles the risk of acquiring HIV, because open sores create an entry point.
In pregnancy the stakes are highest: untreated maternal syphilis harms the baby in 50 to 80% of cases, including stillbirth, prematurity, newborn death, or congenital infection WHO. 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. A true penicillin allergy in pregnancy is handled by desensitizing the patient and treating with penicillin anyway, because there is no safe substitute that protects the baby.
Preventing syphilis going forward
After treatment, prevention is about not getting reinfected. Correct and consistent condom use, mutual monogamy with a partner who has tested negative, and routine screening if you're at ongoing risk are the foundation. Because timing matters, our guide on when to test after exposure explains why a test soon after exposure can read negative and needs repeating.
For people at higher risk, DoxyPEP is an option worth discussing: a single 200 mg dose of doxycycline taken within 72 hours after sex reduced syphilis acquisition by about 73% in trials, and the CDC recommends it through shared decision-making for men who have sex with men and transgender women who've had a bacterial STI in the past 12 months CDC DoxyPEP, 2024.
Syphilis keeps climbing. Roughly 53,000 primary-and-secondary cases were reported in 2023, with rates several times higher in some states than the national average, so staying on a screening schedule is reasonable if you're sexually active with new or multiple partners.
When to see a clinician
Call your clinician if your fever and aches don't settle within about a day, if they're severe, or if you're pregnant and developing contractions. Seek emergency care for true allergy signs such as hives, swelling of the lips or throat, trouble breathing, or for any new vision changes, severe headache, confusion, or weakness, which can signal eye or nervous-system involvement. And come back for your scheduled titer checks even if you feel completely well.
- Fever and aches lasting longer than about a day or feeling severe.
- Any sign of a true allergic reaction: hives, lip or throat swelling, wheezing, lightheadedness.
- New vision changes, severe headache, confusion, weakness, or hearing loss.
- Pregnancy with fever, contractions, or reduced fetal movement after a shot.
- A follow-up titer that rises instead of falling, which can mean reinfection or failure.