Is it normal to have fever after taking antibiotics? Understanding causes and complications

October 12, 2025 •

5 min read

A low-grade fever can occur during the first 48–72 hours after starting a new antibiotic as the medicine begins to work. Therefore, understanding the difference between a normal reaction and a more concerning complication is crucial if you have a fever after taking antibiotics.

Quick Summary

An initial low-grade temperature is often a normal part of the healing process on antibiotics. However, a fever that rises, persists beyond 72 hours, or is accompanied by other symptoms may indicate a more serious issue such as drug fever, an allergic reaction, or treatment failure.

Key Points

Initial Fever is Common: A low-grade fever for the first 48-72 hours is often a normal sign that the antibiotic is starting to fight the infection and clear bacteria.
Persistent Fever is a Concern: A fever that lasts more than three days, worsens, or returns after disappearing suggests potential problems, such as an ineffective antibiotic or a complication.
Watch for Drug-Induced Fever: The antibiotic itself can cause a fever, sometimes weeks into treatment, which usually subsides within 72 hours of discontinuing the medication.
Recognize the Jarisch-Herxheimer Reaction: This is a specific reaction causing a temporary fever and flu-like symptoms after treating infections like syphilis or Lyme disease, and it's not an allergy.
Seek Help for Red Flags: New rashes, very high temperatures (103°F+), rapid heart rate, or severe diarrhea alongside a fever are serious warning signs that require immediate medical attention.
Distinguish Fever from Infection: Be aware that a pre-existing or co-existing viral infection can also cause a fever, which will not be resolved by antibiotics.
Follow Up on Symptoms: If improvement is not noticeable after 2-3 days, or if new symptoms appear, it's essential to follow up with your doctor for re-evaluation.

The Normal Response: When a Fever is Expected

When you start taking an antibiotic for a bacterial infection, it is not uncommon for your body to have a low-grade fever during the first 48 to 72 hours. This initial temperature spike is part of the normal healing process and indicates the medication is working. As the antibiotic attacks and kills the bacteria, the dying pathogens release toxins and other substances into the bloodstream. This triggers an inflammatory response as your immune system mounts its defense, which can cause a temporary, mild fever. For example, up to 60% of patients with pneumonia may see no temperature drop on day one because of bacterial toxins. By day two, however, a downward trend of at least 1°F is a positive sign that the treatment is effective.

During this initial period, you might expect the following:

Your fever may not disappear immediately, but it should not get significantly worse.
Other infection symptoms, such as pain or fatigue, may begin to improve.
Your fever should begin to trend downward and eventually subside within a few days of starting the medication.

Causes of Fever While on Antibiotics

If a fever persists, worsens, or returns after an initial improvement, it could be due to several factors. These reasons for an ongoing fever can range from common issues to more serious complications that require immediate medical attention.

Drug-Induced Fever

In some cases, the antibiotic itself is the cause of the fever. Known as drug-induced fever, this is an adverse reaction where the medication triggers a febrile response. It can occur with many antibiotics, especially beta-lactams (penicillins and cephalosporins), minocycline, and sulfonamides. A drug fever typically appears 7 to 10 days after starting the medication, though it can happen sooner. A key sign of a drug fever is that the temperature often resolves within 48 to 72 hours of stopping the offending drug.

The Jarisch-Herxheimer Reaction

This is an acute, short-term inflammatory response that occurs when a large number of spirochete bacteria (like those causing syphilis or Lyme disease) are killed off rapidly by antibiotics. The release of bacterial lipoproteins and endotoxin-like substances triggers a systemic inflammatory reaction that includes fever, chills, muscle aches, and worsening of the existing rash. The reaction usually begins within hours of the first antibiotic dose and subsides within 24 hours.

Allergic Hypersensitivity

An allergic reaction to an antibiotic can manifest as a fever. This can happen as part of a mild allergic response or a more severe, life-threatening condition like Drug Reaction with Eosinophilia and Systemic Symptoms (DRESS) or Stevens-Johnson syndrome. A fever with a new rash after starting antibiotics is a combination that should be evaluated by a healthcare provider immediately.

Ineffective or Resistant Antibiotic

If the fever persists beyond the initial 48-72 hour window, it may be a sign that the prescribed antibiotic is ineffective against the specific bacteria causing the infection. This can be due to antibiotic resistance or if the infection is more complex or has spread. In these cases, the healthcare provider will likely need to re-evaluate the treatment plan.

Coexisting Viral Infection

Antibiotics are only effective against bacterial infections and have no effect on viruses. If you have a fever from a viral co-infection, such as influenza, the fever will not improve with the antibiotic and must resolve on its own.

When to Be Concerned: Red Flags to Watch For

While a mild, short-lived fever is often part of recovery, certain signs and symptoms warrant contacting a healthcare professional immediately. The following is a list of red flags to watch for:

Fever persistence: If the fever lasts longer than 72 hours, especially if it remains above 100.4°F (38°C).
Increasing fever: A fever that rises after initially improving or gets significantly higher.
High fever: A temperature of 103°F (39.4°C) or higher at any point during antibiotic treatment.
New rash: The development of a new rash, particularly one that is red, painful, or blistering.
Systemic symptoms: Other severe symptoms such as difficulty breathing, rapid heart rate (>120 bpm), or a feeling of overall illness.
New or worsening pain: The onset of flank or back pain, which could indicate the infection has spread to the kidneys, as seen in pyelonephritis.
Severe diarrhea: Watery or bloody diarrhea, especially if it's accompanied by a fever, can indicate a serious gastrointestinal issue like C. difficile.

Understanding the Cause: Comparing Normal vs. Concerning Fever


Feature	Normal Initial Fever (Expected)	Concerning Fever (Requires Attention)
Onset	Occurs within the first 48-72 hours of starting antibiotics.	Starts or returns after the first few days, or appears 7-10 days into treatment (drug fever).
Duration	Lasts a few days and then resolves as the infection clears.	Persists beyond 72 hours, rises, or comes and goes over a week or more.
Associated Symptoms	May include fatigue and general illness, which should improve over time.	Accompanied by new rash, swelling, rapid heart rate, confusion, or severe gastrointestinal issues.
Temperature Pattern	Often a mild, low-grade temperature.	Can be high (103°F or more) or a remittent fever that spikes and dips.
Immune Markers	Part of a localized or systemic inflammatory response.	May show additional signs like a high eosinophil count in drug-induced fever.

Conclusion

While it can be normal to have fever after taking antibiotics, particularly within the first few days of treatment, it is important to pay close attention to its duration and accompanying symptoms. An initial, low-grade fever is often a sign that the medication is beginning to work and the body is reacting to the clearing of the infection. However, a fever that becomes higher, lasts longer than 72 hours, or reappears with new symptoms is a clear signal that something is wrong. Whether it's a drug-induced fever, an allergic reaction, or a sign of an ineffective antibiotic, persistent fever warrants a prompt medical evaluation to ensure the proper course of treatment is followed and to prevent potential complications. Always consult a healthcare provider with any concerns about a fever during antibiotic treatment. More information can be found on reputable medical sites like the National Institutes of Health.

Visit the National Institutes of Health (NIH) website for further information on adverse drug reactions.

Frequently Asked Questions

A fever should typically begin to subside within 48 to 72 hours of starting an effective antibiotic. If your fever persists beyond this period or gets worse, you should contact your doctor.

Yes, antibiotics can cause a fever. This is known as drug-induced fever and can be a side effect or an allergic reaction. It is a particular risk with certain types, like beta-lactams and sulfonamides.

The Jarisch-Herxheimer reaction is a short-term, acute inflammatory response that occurs when a large number of bacteria are killed off quickly by an antibiotic, releasing toxins. It causes flu-like symptoms, including fever, and is commonly associated with treating spirochete infections like syphilis and Lyme disease.

Yes, a new rash combined with a fever after taking antibiotics is a red flag and could indicate a serious drug hypersensitivity reaction. You should seek immediate medical attention.

A returning fever is a sign that the antibiotic may be ineffective, the infection has progressed, or you have a new infection. It requires a medical re-evaluation to determine the cause.

Yes. Antibiotics do not treat viruses. If you have a concurrent viral illness like the flu, your fever will persist until the viral infection resolves on its own, regardless of the antibiotic treatment.

Contact a doctor if your fever lasts more than 72 hours, rises significantly, exceeds 103°F (39.4°C), or is accompanied by new or severe symptoms like a rash, rapid heart rate, difficulty breathing, or severe diarrhea.