Fever as a Side Effect of Phenytoin
Fever associated with phenytoin is uncommon but clinically significant, occurring as part of serious hypersensitivity reactions rather than as an isolated adverse effect. The FDA drug label specifically mentions fever in the context of Anticonvulsant Hypersensitivity Syndrome (AHS) and lymphadenopathy with serum sickness-like symptoms 1.
Incidence and Clinical Context
Fever from phenytoin does not occur as an isolated side effect but rather as a component of serious hypersensitivity syndromes. The available evidence does not provide a specific percentage for fever alone, but rather describes it within the following contexts:
Anticonvulsant Hypersensitivity Syndrome (AHS)
- Characterized by the triad of fever, rash, and lymphadenopathy with multiorgan involvement
- Described as "rare" in the FDA label 1
- Potentially fatal when it occurs
- Mechanism remains unknown
Lymphadenopathy with Serum Sickness-Like Symptoms
- Fever occurs alongside rash and liver involvement 1
- No specific incidence rate provided in the drug label
Cutaneous Hypersensitivity Reactions
- In a Singapore hospital study, fever was reported in 4.8% of confirmed phenytoin allergy cases 2
- However, this represents fever among patients who already had confirmed allergic reactions, not the general population taking phenytoin
- The majority (92.9%) of allergic reactions were cutaneous, with fever being a less common manifestation
Important Clinical Caveats
The absence of specific incidence data for fever as an isolated symptom reflects that phenytoin-induced fever is clinically meaningful only when part of a hypersensitivity syndrome. When fever occurs with phenytoin:
- Always evaluate for accompanying symptoms: rash, lymphadenopathy, hepatic involvement
- Consider immediate discontinuation if hypersensitivity syndrome is suspected 1
- Do not rechallenge if serious hypersensitivity occurred
- Monitor for progression to Stevens-Johnson syndrome or toxic epidermal necrolysis, which occurred in 14.3% and 2.4% respectively of allergic reactions in one series 2
Risk Factors
Certain populations may have increased risk of hypersensitivity reactions (which include fever):
- HLA-B*15:02 carriers (particularly Asian ancestry) have increased risk of serious cutaneous reactions 3
- Black patients may have increased risk of hypersensitivity reactions including hepatotoxicity 1
The key clinical message: fever with phenytoin should prompt immediate evaluation for hypersensitivity syndrome rather than being dismissed as a common side effect, as it signals potentially life-threatening reactions in a small but significant subset of patients.