Can the antiepileptic drug phenytoin cause fever?

Can Phenytoin Cause Fever?

Yes, phenytoin can definitely cause fever as part of drug-induced hypersensitivity reactions, and this is a well-recognized adverse effect that clinicians must actively monitor for.

Mechanism and Clinical Presentation

Phenytoin causes fever primarily through drug hypersensitivity reactions rather than direct pharmacologic effects. According to the FDA drug label, dermatological manifestations are "sometimes accompanied by fever" 1. The 2008 Critical Care Medicine guidelines explicitly recognize that any drug can cause fever due to hypersensitivity, with phenytoin specifically listed among the agents most commonly associated with drug-induced fever 2, 3.

Key Clinical Syndromes

Drug-Induced Hypersensitivity Syndrome (DIHS)/DRESS:

• Classic triad: fever, rash, and visceral organ involvement (particularly liver) 4, 5
• Onset typically 1-12 weeks after starting phenytoin 5
• Mean lag time of 21 days (median 8 days) between drug initiation and fever 2
• Fever may persist for 1-7 days after discontinuing phenytoin 2

Anticonvulsant Hypersensitivity Syndrome (AHS):

• The FDA label specifically warns about this syndrome, which includes fever along with arthralgias, eosinophilia, liver dysfunction, lymphadenopathy, and rash 1

Diagnostic Approach

Look for these specific features:

• Temporal relationship: Fever developing days to weeks after phenytoin initiation
• Associated symptoms: Rash (morbilliform most common), pruritus, sore throat, jaundice
• Laboratory abnormalities: Eosinophilia, elevated liver enzymes, leukopenia
• Lymphadenopathy (may occur with or without serum sickness-like symptoms) 1

Important caveat: Rash occurs in only a fraction of cases, and eosinophilia is uncommon, so absence of these findings does not exclude drug-induced fever 2.

Clinical Significance and Management

Research demonstrates that phenytoin-associated fever has real clinical consequences. In a prospective study of intracerebral hemorrhage patients, phenytoin was independently associated with more fever (P=0.03) and worse outcomes 6.

Immediate management steps:

1. Discontinue phenytoin immediately - this is essential for good outcomes 5
2. Switch to alternative antiepileptic (levetiracetam or valproate are safer alternatives per current guidelines 7)
3. Supportive care: hydration, antihistamines, H2-blockers, topical corticosteroids 5
4. Systemic corticosteroids for severe cases - a 2024 case report demonstrated rapid improvement within 48 hours of dexamethasone initiation 4

Critical Pitfalls to Avoid

Cross-reactivity warning: Patients with phenytoin hypersensitivity may react to other aromatic antiepileptics (carbamazepine, phenobarbital, primidone, lamotrigine) 8, 5. A documented case showed progressive worsening when switching from phenytoin to divalproex to phenobarbital, ultimately resulting in fulminant hepatic failure 8.

Do not rechallenge patients who had anaphylaxis or toxic epidermal necrolysis 2.

Monitor for severe cutaneous reactions: Stevens-Johnson syndrome and toxic epidermal necrolysis can be fatal 1. The HLA-B*15:02 allele increases this risk 9.

Bottom Line for Clinical Practice

Phenytoin-induced fever is a recognized adverse effect occurring through hypersensitivity mechanisms. When fever develops in a patient on phenytoin—especially with rash, organ dysfunction, or hematologic abnormalities—immediately discontinue phenytoin and switch to an alternative antiepileptic (levetiracetam or valproate preferred). Early recognition and drug discontinuation are critical to prevent progression to life-threatening complications including fulminant hepatic failure and severe cutaneous reactions.