The condition most people label foot fungus covers two distinct infections that researchers have tracked across dozens of trials: tinea pedis on the skin and onychomycosis in the nail. Both arise from dermatophytes, chiefly Trichophyton rubrum, and both thrive where moisture lingers. Clinical studies separate them because their course and response to treatment differ in measurable ways.

Patterns observed in patient cohorts

Interdigital tinea pedis, the version between the toes, shows up in trials as itching, maceration, fissuring, and fine silvery scales. The moccasin or hyperkeratotic form spreads across the sole with thicker, powdery scaling and less acute itch. Onychomycosis adds nail-plate thickening, yellow-white discoloration, and brittleness that progresses from the distal edge. These descriptions come from consistent physical-examination criteria used in the randomised trials that underpin current guidelines.

Diagnosis in research settings almost always includes potassium-hydroxide microscopy plus culture confirmation before enrolment. Without that step, studies risk including non-fungal mimics such as psoriasis or contact dermatitis. Real-world confirmation often stops at clinical appearance, which explains why some over-the-counter courses fail: the wrong organism or no fungus at all was present.

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What the treatment trials actually measured

A 2022 systematic review pooled seven randomised controlled trials with 1,042 participants who had culture-proven tinea pedis. Topical terbinafine produced a relative risk of resolution 3.9 times that of placebo. Butenafine showed a similar advantage. Both belong to the allylamine class and act by blocking ergosterol synthesis earlier in the pathway than the azoles. The same review found oral terbinafine modestly superior to oral itraconazole on the same endpoint.

For onychomycosis the arithmetic shifts. Complete cure, defined as both mycological clearance and visible nail normalisation, remains harder to achieve because the nail grows slowly and drug penetration varies. Network meta-analyses and head-to-head trials repeatedly place continuous terbinafine ahead of continuous or pulsed itraconazole on mycological cure rates, with differences on the order of 70–81 percent versus 63 percent at 48–72 weeks. Fluconazole trails further behind in the same datasets.

These figures come from trials that pre-specified culture endpoints and followed patients long enough for nail outgrowth. Many still carried unclear risk of bias on allocation concealment or had greater than 20 percent attrition, which lowers the GRADE certainty to moderate. No trial design can eliminate the reality that reinfection risk stays high once treatment stops if footwear and hygiene habits remain unchanged.

Practical steps that align with the evidence

Keep feet dry and change socks when damp. Alternate shoes so each pair dries fully between wears. In shared wet areas, barrier footwear reduces exposure. These measures appear in every prevention arm of the studies because the organisms require moisture to establish.

Over-the-counter allylamine creams remain first-line for uncomplicated skin involvement. Apply beyond the visible margin and continue at least one week after visible clearing. Nail involvement usually needs laboratory confirmation before oral therapy begins, both to document the pathogen and to exclude contraindications such as active liver disease.

Combination approaches surface in newer trials when single-agent results plateau. Adding a topical to an oral course or using a keratolytic to thin the nail plate before topical application can improve penetration, though the incremental gain varies with disease severity and patient adherence.

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Recurrence data are thinner than cure data. Five-year follow-up in one large cohort showed lower relapse with terbinafine than with itraconazole, yet roughly one in five patients still experienced return of infection. The studies do not isolate whether the return represented incomplete eradication or fresh acquisition; both mechanisms operate.

Complementary agents such as ozonized oils or tea-tree preparations show in-vitro activity and occasional small-trial equivalence to azoles for skin disease, but they lack the large, pre-registered trials that would let clinicians assign precise effect sizes. Patients who prefer them should still monitor for lack of improvement within the same two-to-four-week window used for conventional topicals.

The design choices that matter most are the ones rarely highlighted in consumer summaries: mandatory culture confirmation at baseline, pre-registered primary endpoints, and follow-up long enough to capture nail regrowth. When those elements are present, the modest advantage of terbinafine over alternatives becomes reproducible. When they are absent, dramatic claims rest on weaker footing. That distinction separates what a study can show from what it can only suggest.