Managing cutaneous fungal infection in DMARD/biologic-treated patients demands attention to both altered presentation and constrained pharmacologic choice — heightened suspicion, liberal confirmatory testing, class-informed risk stratification, and antifungal selection that accounts for DMARD- and JAK-specific interactions.

 Part I — Diagnostic Challenges

Atypical and Masked Presentations

Immunosuppression can blunt the inflammatory response that produces classic dermatophyte morphology, yielding atypical, minimally inflamed, or “id-like” lesions that deviate from textbook presentations clinicians rely on for bedside diagnosis.

Mimicry and Overlap

• Drug-induced mimics: DMARD/biologic-induced eruptions — psoriasiform or eczematous reactions to biologic therapy, or methotrexate-related skin changes — can closely resemble true fungal infection, creating a genuine bedside diagnostic trap.

• Psoriasis overlap: In psoriasis, nail disease itself elevates baseline onychomycosis prevalence (~18.0%), and immunosuppression may aggravate underlying mycotic infection, further blurring diagnosis.

The Indian Recalcitrant Dermatophytosis Problem

Trichophyton indotineae infection can present deceptively — appearing mild or being misread as an eczema or psoriasis flare in an immunosuppressed patient — delaying correct diagnosis and therapy. This regional epidemic of terbinafine-resistant dermatophytosis adds diagnostic uncertainty not captured by standard clinical algorithms.

Confirmatory Testing

Given the unreliability of clinical diagnosis alone in this population, KOH microscopy, fungal culture, and PCR-based identification should be used liberally rather than reserved for treatment failure. Biopsy is warranted when lesions are atypical, deep, or unresponsive to empiric therapy.

Part II — Treatment Challenges

Class-Specific Risk Informs Urgency

• csDMARDs (methotrexate): Modest infection signal overall; fungal skin infection is not a labeled hallmark adverse event.

• TNF inhibitors: Superficial infections (dermatophytosis, onychomycosis, candidiasis) and, uncommonly (<0.5%/year), invasive/opportunistic mycoses such as histoplasmosis.

• IL-17 inhibitors: The standout class for mucocutaneous candidiasis (WHO ROR 12.28 for cutaneous candidiasis); usually mild and manageable.

• JAK inhibitors: Low but real opportunistic risk; triazole–CYP interactions become the dominant management concern.

Antifungal Selection Under Polypharmacy

• Terbinafine: First-line for dermatophyte infection across nearly all DMARD backgrounds; contraindicated in active liver disease, requires baseline/periodic LFTs.

• Fluconazole: Preferred for candidal disease; rare hepatotoxicity and QT risk, but no unique methotrexate-specific hazard.

• Itraconazole: Least attractive option — CYP3A4 interaction burden, hepatotoxicity, and CHF/negative-inotropy risk.

Terbinafine Resistance Reopens the Dilemma

Standard dosing may fail against resistant T. indotineae strains, forcing dose escalation, prolonged courses, or a shift to itraconazole/fluconazole — agents least compatible with concurrent DMARD or JAK-inhibitor therapy. This failure mode reopens the hepatotoxicity and CYP-interaction concerns first-line terbinafine would otherwise avoid.

JAK Inhibitors: A Distinct Interaction Landscape

Unlike methotrexate’s hepatotoxicity-centered risk, JAK inhibitor combinations are dominated by triazole–CYP interactions. Fluconazole, for example, should be avoided with abrocitinib, as CYP2C19/CYP2C9 inhibition raises drug exposure and may necessitate dose reduction.

Continue or Interrupt Therapy?

Uncomplicated superficial infection responding to standard therapy is not, by itself, reason to hold biologic or JAK therapy — including IL-17-associated candidiasis, which is typically mild. Interruption is reserved for fever, systemic illness, rapidly progressive or invasive disease, treatment failure, or unsafe drug interactions. JAK inhibitors warrant a lower threshold for reassessment given reports of atypical dermatophytosis under JAK-blunted host defense. Restart is generally guided by clinical resolution rather than a fixed interval.