Drug Fever

Definition and Epidemiology

Causative Drugs

Mechanisms

1. Hypersensitivity reaction - The most common mechanism. Drugs act as haptens, binding to host proteins to form drug-hapten conjugates recognized as "modified self." This can trigger any of the four types of hypersensitivity:
   • Type I (IgE-mediated): mast cell degranulation, anaphylaxis spectrum
   • Type II: IgG/IgM antibodies against drug-modified host cells (e.g., hemolytic anemia)
   • Type III: Immune complex deposition - serum sickness pattern with urticaria, arthralgia, lymphadenopathy, glomerulonephritis, and fever
   • Type IV: Cell-mediated (contact dermatitis, DRESS syndrome)
2. Pyogenic effect - Some drugs or their metabolites directly stimulate pyrogenic mediator release independent of immune mechanisms.
3. Disturbed thermoregulation - Central effects on the hypothalamic thermoregulatory center.

Clinical Features

• Onset: Fever typically appears several weeks after initiating the drug, but can occur at any point during therapy
• Temperature: Can be high - often spiking without a definitive infectious source
• Patient appearance: Patients may appear paradoxically well despite significant fever (a key clue)
• Associated findings:
  • Peripheral eosinophilia - present in ~20% of cases
  • Skin rash - present in ~20% of cases
  • These are important diagnostic clues when present
• Resolution: Fever typically resolves within 48 hours of discontinuing the causative agent

DRESS Syndrome (Drug Reaction with Eosinophilia and Systemic Symptoms)

• Rash developing late (2-8 weeks after drug initiation)
• Fever (>38°C)
• Eosinophilia or activated lymphocytosis
• Multiorgan involvement (hepatic, renal, pulmonary)
• Lymphadenopathy, hepatomegaly
• Can progress to Stevens-Johnson syndrome if the drug is not promptly stopped

Diagnosis

1. Take a thorough medication history - include all prescription, OTC, and herbal drugs. New drugs added in the weeks prior to fever onset are prime suspects.
2. Rule out infection - blood, urine, and other cultures as clinically indicated; imaging if needed.
3. Look for clues: eosinophilia, rash, the paradox of a sick-looking temperature chart with a well-appearing patient.
4. Stop the suspect drug - if no infectious source is found and the patient is stable, discontinuing the likely offending agent is both diagnostic and therapeutic. Fever resolves within ~2 days.
5. Rechallenge - confirming evidence can be sought by cautiously re-exposing the patient to the drug. However, rechallenge should only be done if the drug is critical to care, and is generally avoided when an immunologic mechanism is suspected.
6. Genetic testing - in some cases (e.g., G6PD deficiency), genotyping can help clarify the mechanism.

Management

• Discontinue the offending drug - the cornerstone of treatment
• Supportive care (antipyretics, fluids) while the fever resolves
• For severe immunologic reactions (DRESS, SJS spectrum): high-dose corticosteroids; in refractory cases - rituximab, infliximab, abatacept, mycophenolate mofetil, or plasmapheresis have been used with variable success
• If the patient requires multiple drugs, discontinue the most suspect agent first, then observe; or stop all non-essential drugs simultaneously
• Note that drugs with long half-lives may take longer than 48 hours for fever to resolve

Key Practical Points

• Drug fever is a common cause of FUO - always include a medication review
• The "paradoxically well" patient with high fever and no source is a classic presentation
• Antibiotics (used to treat infection) are the most commonly implicated class - do not assume fever during antibiotic therapy means treatment failure
• Phenytoin is the most common culprit in neurology/NICU settings
• Eosinophilia and/or rash in ~1 in 5 patients - absence does not exclude the diagnosis
• Fever resolving within 2 days of drug cessation is strongly confirmatory