What is Herxing?: An In-Depth Look at the Jarisch-Herxheimer Reaction

October 6, 2025 •

4 min read

First documented in the late 19th century while treating syphilis, the Jarisch-Herxheimer reaction, or herxing, is a temporary flare-up of symptoms that can occur after starting antimicrobial therapy. This phenomenon can be alarming but is often considered an indication that the treatment is effectively killing off the targeted pathogens.

Quick Summary

A Jarisch-Herxheimer reaction, or herxing, is a temporary flare of symptoms resulting from the immune system's inflammatory response to toxins released by rapidly dying microbes during treatment. It is not an allergic reaction to medication.

Key Points

Microbial Die-Off: Herxing is a temporary immune response to toxins released by rapidly dying pathogens during treatment.
Symptom Flare: Common symptoms include fever, chills, fatigue, and headaches, which can worsen existing illness symptoms.
Positive Indication: Feeling worse before you feel better can be a sign that the antimicrobial treatment is effectively working.
Not an Allergy: A herx reaction is distinct from a medication allergy; misdiagnosis can be dangerous.
Supportive Care is Key: Hydration, rest, and supporting detoxification pathways can help manage symptoms and make the reaction more tolerable.
Severity Varies: The duration and intensity of herxing depend on the infection type and individual factors, lasting from hours to weeks.

What Causes a Herxheimer Reaction?

The precise mechanism of the Jarisch-Herxheimer reaction, commonly referred to as herxing, is not yet fully understood, but the prevailing theory centers on the release of bacterial toxins and the subsequent immune response. When antibiotics or other antimicrobial treatments rapidly kill off pathogens, these microorganisms release endotoxins, lipoproteins, and other toxic byproducts into the bloodstream.

This sudden influx of toxic waste triggers an acute systemic inflammatory response from the body's immune system. This response involves the release of pro-inflammatory cytokines, such as tumor necrosis factor alpha (TNF-α) and interleukins (IL-6, IL-8), which are responsible for many of the flu-like symptoms associated with herxing. The inflammatory cascade can overwhelm the body’s detoxification pathways, particularly the liver and kidneys, causing symptoms to intensify temporarily.

Infections and Treatments That Can Trigger Herxing

While famously associated with syphilis treatment, the herxheimer reaction can occur with a variety of spirochetal infections and other microbial imbalances. Common triggers include:

Spirochetal Infections: These include syphilis (caused by Treponema pallidum), Lyme disease (Borrelia burgdorferi), leptospirosis, and relapsing fever.
Chronic Infections: The term is often used more broadly in functional medicine to describe similar die-off reactions when treating chronic, low-grade infections like Small Intestinal Bacterial Overgrowth (SIBO), Candida overgrowth, and other imbalances.
Antimicrobial Therapies: The reaction can be triggered by a wide range of therapies, including prescription antibiotics (like penicillin and doxycycline), herbal antimicrobial supplements (e.g., oregano oil, cryptolepis), and even some forms of oxygen therapy.

Signs and Symptoms of Herxing

The symptoms of a herxheimer reaction can range from mild to severe and often mimic a worsening of the original illness or the flu. The severity can correlate with the amount of dead bacteria and toxins being released.

Common physical symptoms include:

Fever and chills
Exacerbated fatigue and lethargy
Muscle and joint pain (myalgia)
Headaches
Gastrointestinal issues, such as nausea, diarrhea, or constipation
Flushing or skin rashes
Swollen lymph nodes

Neurological or cognitive symptoms may include:

Brain fog or confusion
Anxiety and mood shifts
Insomnia

In rare, severe cases, more serious manifestations can occur, such as a significant drop in blood pressure (hypotension), tachycardia, or organ dysfunction, which require immediate medical attention.

Herxheimer Reaction vs. Allergic Reaction: A Comparison

It is crucial to differentiate a herx reaction from a true allergic reaction to the medication, as the management approach is entirely different. Mistaking an allergy for herxing can lead to life-threatening complications.


Feature	Herxheimer Reaction	Allergic Reaction (e.g., to Penicillin)
Cause	Immune response to microbial toxins released during pathogen die-off.	Hypersensitivity immune response to the drug itself, not the infection.
Onset	Typically hours to days after treatment begins. Can be delayed in chronic illnesses like Lyme.	Can occur immediately or minutes after exposure.
Duration	Usually temporary and self-limiting. For syphilis, typically 12-24 hours; for chronic illnesses, can last days or weeks.	Persists until the drug is out of the system and, in severe cases, requires emergency treatment.
Symptoms	Flu-like symptoms, fever, chills, myalgia, fatigue, headache, worsening of existing symptoms or rash.	Generalized urticaria (hives), swelling, itching, and in severe cases, anaphylaxis (difficulty breathing, swelling of airways).
Outcome	Often indicates treatment is working, with symptoms resolving over time.	Requires cessation of the medication and specific medical intervention; potentially fatal if untreated.

Managing the Herxheimer Reaction

While herxing can feel intense, it is typically a sign of progress, and its symptoms can often be managed with supportive care. The goal is to ease discomfort and aid detoxification without halting the effective treatment. Consult a healthcare provider before making any changes to your medication protocol.

Hydration: Increase your fluid intake with plenty of filtered water and herbal teas to help flush toxins from the body and support kidney function.
Rest: The body needs adequate rest to perform detoxification processes. Listen to your body and allow for extra sleep during a flare.
Dose Adjustment: Your doctor may recommend a reduction in your treatment dosage to slow the rate of microbial die-off, making the reaction more tolerable.
Support Detoxification Pathways: Therapies that support liver and lymphatic drainage, like gentle exercise, dry brushing, Epsom salt baths, or saunas, can be beneficial.
Gastrointestinal Support: Probiotics can help restore gut health, and binders like activated charcoal can trap and remove toxins from the gut. Binders should be taken at least one hour away from medications and food.
Diet: A nutrient-dense, whole foods diet rich in antioxidants and low in inflammatory foods can help support the body's systems.

Conclusion

Herxing, or the Jarisch-Herxheimer reaction, is a temporary, sometimes challenging, but often beneficial side effect of effective antimicrobial treatment. It indicates that the therapeutic strategy is successfully killing off the target pathogens and that the body's immune system is actively working to clear the debris. By understanding the mechanism and employing supportive care strategies under medical supervision, patients can better navigate this phase of treatment. While managing symptoms is key, the experience of a herx reaction can be a reassuring sign that healing is underway.

For more clinical details on the Jarisch-Herxheimer reaction, especially in relation to syphilis, visit the National Institutes of Health (NIH) StatPearls database.

Frequently Asked Questions

The duration of a herx reaction is highly variable. In acute infections like syphilis, it can resolve within 12-24 hours. For chronic conditions like Lyme disease, it might last for days or weeks and can be cyclical.

Yes, herxing is generally considered a good sign that the treatment is working effectively and that the body is eliminating pathogens. The temporary worsening of symptoms indicates that the therapeutic process is underway.

Herxing is a systemic inflammatory response to microbial toxins, typically involving flu-like symptoms. An allergic reaction is a hypersensitivity to the drug itself, often presenting with hives, itching, or swelling. Allergic reactions do not improve by continuing the medication.

The primary management strategy is supportive care. This includes resting, increasing fluid intake, and potentially adjusting the medication dosage under a doctor's guidance. Other support measures like detoxification baths and supplements may also help.

There is no definitive way to prevent a herx reaction, as it is a natural immune response. However, a healthcare provider can manage its severity by starting with a low dose of medication and gradually increasing it, allowing the body to adjust more slowly.

Herxing is most commonly associated with spirochetal infections, such as syphilis and Lyme disease, when treated with antibiotics. It can also occur when treating other infections or microbial imbalances, like Candida or SIBO, in a broader context.

Natural supportive strategies include staying well-hydrated, using activated charcoal or other binders to help trap toxins, taking Epsom salt baths, and ensuring adequate rest. Anti-inflammatory foods and antioxidant supplements like glutathione can also help.

While most herx reactions are mild and self-limiting, some severe cases, particularly in patients with advanced syphilis or certain chronic infections, can lead to dangerous drops in blood pressure and organ damage. It is vital to seek medical attention for severe symptoms.