Drug Fever

How drug fever happens

Medicines can cause fever by several mechanisms:

• Hypersensitivity (allergic-type) reactions: the most common; the immune system reacts to the drug.
• Effects on temperature control: some drugs alter heat production or heat loss, such as stimulants and anticholinergics.
• Drug-administration reactions: contaminants or reactions at infusion sites.
• Direct pharmacological effects: such as cell breakdown after chemotherapy (tumor lysis) or the Jarisch-Herxheimer reaction after treating syphilis.
• Idiosyncratic reactions: such as malignant hyperthermia during anesthesia and neuroleptic malignant syndrome.

Common medicines that cause drug fever

Almost any medicine can cause drug fever, including ones taken safely for years.

Signs that suggest drug fever

• fever starting 7 to 10 days after a new drug, or within hours to days on re-exposure
• feeling better than expected for the fever level
• a pulse that is lower than expected for the temperature (relative bradycardia), in some cases
• a rash in some people
• raised eosinophils (a type of white blood cell) or mildly abnormal liver tests
• no clear infection despite tests
• fever that persists while an infection is otherwise improving on antibiotics

Fevers may be low-grade or very high, up to 40 °C (104 °F) or more, and can be constant or come and go.

Diagnosis

Drug fever is a diagnosis of exclusion. Doctors:

1. Review medicines, including recent starts, dose changes and over-the-counter products.
2. Look for infection and other causes with an exam, blood counts, cultures, urine tests and imaging as needed.
3. Stop or switch the suspected drug when it is safe to do so. A fever that settles within 48 to 72 hours strongly supports drug fever.

Deliberately restarting the drug to confirm the diagnosis is usually avoided because reactions can be stronger the second time.

Treatment

• Stop the suspected medicine and replace it with an unrelated alternative if needed.
• Supportive care: fluids and fever reducers such as acetaminophen (paracetamol) for comfort.
• Treat severe reactions: serious drug reactions such as Stevens-Johnson syndrome or DRESS syndrome need hospital care.
• Record the reaction: make sure it is listed in your medical records as a drug reaction or drug allergy, so the medicine is avoided in future.

Outlook

Once the medicine is stopped, fever usually resolves within 72 hours, though it can take longer for drugs that stay in the body for a long time. Most people recover completely. Drug fever can recur quickly if the same drug, or sometimes a closely related one, is given again.

Frequently asked questions

What is drug fever?

Drug fever is a fever caused by a medicine rather than by infection or another illness. It usually begins about a week to 10 days after starting the drug and goes away within a few days of stopping it.

Which medications commonly cause fever?

Beta-lactam antibiotics such as penicillins and cephalosporins, sulfa antibiotics, anti-seizure medicines such as phenytoin and carbamazepine, allopurinol, heparin, and some heart medicines are among the most common causes.

How long does drug fever last?

Fever typically resolves within 48 to 72 hours after stopping the responsible medicine. If a fever continues longer, doctors look again for infection or other causes, or consider a slowly cleared drug.

Should I stop my medicine if I get a fever?

Not without medical advice. Fever is much more often caused by infection, and stopping some medicines suddenly is dangerous. Contact your prescriber, who can assess you and decide whether to stop or switch the drug.

How do doctors know a fever is caused by a drug?

They rule out infection and other causes with examination and tests, then stop the suspected drug. If the fever disappears within 2 to 3 days, drug fever is the likely cause. Clues include feeling relatively well and a recently started medicine.