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Long-term LymeJarisch-Herxheimer Reaction
What a “herx” reaction is, how common it is with Lyme treatment, how it differs from side effects and allergy, and when to call a doctor.
Quick answer
The Jarisch-Herxheimer reaction, often called a “herx,” is a short flare of fever, chills, headache and aches that can start within hours of beginning antibiotics for infections like Lyme disease or syphilis. Medical sources say it usually clears within 24 hours. It is not an allergy, but tell your provider about any reaction.
Medical references describe the Jarisch-Herxheimer reaction as a short-lived inflammatory reaction that can follow the start of antibiotics for infections caused by spiral-shaped bacteria (spirochetes), including Lyme disease and syphilis. It usually begins within a few hours of the first dose and resolves within 24 hours. CDC’s syphilis treatment guidance says it is a reaction to treatment, not an allergy to the medicine, and that patients should be told it can happen.
What it is and why it happens
Lyme disease is caused by Borrelia bacteria, a type of spirochete. When antibiotics start killing these bacteria, the breakdown of the bacteria releases substances that trigger a burst of inflammation. StatPearls, a peer-reviewed medical reference, explains that the exact mechanism is not fully understood, but immune signaling chemicals rise during the reaction and the severity generally tracks how many bacteria are present.
The reaction was first described in syphilis, where it is most common. It also occurs with Lyme disease, leptospirosis and relapsing fever.
Symptoms and timing
According to StatPearls, symptoms can include:
- Fever, chills and shaking
- Headache
- Muscle aches
- Nausea or vomiting
- Flushing and a fast heartbeat
- Low blood pressure
- A temporary flare of a skin rash
Timing is the key feature. The reaction typically starts within a few hours of the first antibiotic dose and goes away within 24 hours. In rare severe cases it can look like a serious bloodstream infection and needs emergency care.
How common is it in Lyme disease?
StatPearls says a Jarisch-Herxheimer reaction has been reported in 7% to 30% of people treated with antibiotics for Lyme disease. Most people recover quickly.
Some people use the word “herx” more loosely, for feeling worse over days or weeks of treatment. That is not what medical sources describe. If you feel worse for more than a day, or keep feeling worse as treatment goes on, tell your provider. It could be a side effect of the medicine, an allergy, a co-infection or something else that needs attention. See co-infections.
Herx, side effect or allergy?
These can feel similar, but they are different. Only your provider can sort out which one you have. Here is how StatPearls describes the differences between a Jarisch-Herxheimer reaction and a drug allergy:
| Jarisch-Herxheimer reaction | Possible drug allergy | |
|---|---|---|
| What it is | Inflammation as bacteria break down | The immune system reacting to the medicine |
| Clues | Fever and chills that start hours after treatment begins | Widespread hives; a history of reactions to the same type of drug, such as penicillin |
| How long | Usually clears within 24 hours | Needs prompt review by your provider |
Antibiotics can also cause ordinary side effects. CDC notes that one reason guidelines prefer the shortest effective course is to reduce side effects such as infectious diarrhea. CDC’s syphilis guidance stresses that the Jarisch-Herxheimer reaction is not an allergy to penicillin.
Call 911 for trouble breathing, swelling of the face, lips or throat, fainting, or confusion. These can be signs of a severe allergic reaction or a serious illness.
Call your provider if you have a fever, chills or worsening symptoms after starting antibiotics, if symptoms last more than a day, if you develop a new rash or hives, or if you are pregnant. CDC notes the reaction may trigger early labor or fetal distress in pregnancy, though this should not delay needed treatment.
Do not stop your antibiotic on your own without talking to your provider.
How it is managed
For mild reactions, StatPearls says care is supportive: fever reducers if your provider says they are okay, and watching symptoms. Antibiotics usually do not need to be stopped. CDC notes that fever reducers can ease symptoms but have not been proven to prevent the reaction. Severe reactions are treated in a hospital.
Before you start treatment, it helps to ask your provider:
- What reactions should I watch for in the first day?
- What can I take for fever or aches, and what should I avoid?
- When should I call you, and when should I go to the emergency room?
See our Lyme disease treatment guide and eating while on antibiotics.
We are collecting real experiences about reactions when starting Lyme treatment from people who have lived it, shared with their permission. As stories come in, they will appear here, clearly marked as personal experience rather than medical advice.
For the bigger picture, see chronic Lyme disease and long-term Lyme. This page is part of step 5 of How to beat Lyme.
Frequently asked questions
How long does a herx reaction last?
Medical sources say a Jarisch-Herxheimer reaction usually starts within a few hours of the first antibiotic dose and resolves within 24 hours. Tell your provider if you feel worse for longer.
Is a herx reaction a sign the antibiotics are working?
It happens as bacteria are killed, but StatPearls notes it does not significantly correlate with how well treatment works (this was studied in syphilis).
Is a Jarisch-Herxheimer reaction an allergy?
No. CDC says it is a reaction to treatment, not an allergy to the medicine. But allergies can also happen, so report any reaction, especially hives or swelling.
Should I stop my antibiotics if I have a herx?
Do not stop on your own. StatPearls says antibiotics usually do not need to be stopped for mild reactions, but your provider should decide.