Toenail Fungus (Onychomycosis): Causes, Treatment &amp

Table of Contents

• What Is Toenail Fungus?
• Symptoms & Appearance
• Causes and Risk Factors
• Confirming the Diagnosis
• Treatment Options Compared
• Over-the-Counter Remedies: What Works?
• Prevention and Recurrence
• When to See a Podiatrist
• Frequently Asked Questions

Toenail fungus is one of the most common and one of the most undertreated conditions we see in our clinic. Patients frequently suffer with discolored, thickened, crumbly nails for years — embarrassed to wear sandals, reluctant to get pedicures, hoping it will resolve on its own. It rarely does. What’s more frustrating: many patients have tried OTC antifungal products for months without improvement, because they’re using the wrong treatment for the wrong reason.

Onychomycosis affects an estimated 10% of the general population and up to 50% of people over 70. In our Michigan practice, we see higher rates in patients with diabetes, athletes, people who frequent public pools or gym showers, and patients with prior tinea pedis (athlete’s foot). Here’s what actually works.

What Is Toenail Fungus (Onychomycosis)?

Onychomycosis is a fungal infection of the nail plate and/or nail bed. The most common causative organisms are dermatophytes — primarily Trichophyton rubrum and Trichophyton mentagrophytes, which account for 90% of cases. Less commonly, yeasts (Candida spp.) and non-dermatophyte molds (NDMs) cause nail infections, particularly in immunocompromised patients or those with prior nail trauma.

Fungal invasion of the nail follows several patterns based on where infection begins. The most common is distal subungual onychomycosis (DSO) — the fungus enters at the hyponychium (the skin under the free edge of the nail) and spreads proximally toward the nail root. White superficial onychomycosis (WSO) and proximal subungual onychomycosis (PSO) are less common variants.

Symptoms and Appearance

Onychomycosis has characteristic visual features that distinguish it from other nail changes:

• Discoloration — yellow-brown-white discoloration is most common. The nail appears dull and opaque rather than its normal translucent pink.
• Thickening — the nail plate and subungual debris (material under the nail) build up, causing the nail to become thickened and difficult to trim.
• Crumbling or brittleness — the nail plate structure degrades. The free edge crumbles when trimmed rather than cutting cleanly.
• Subungual debris — friable white-brown material accumulates between the nail plate and nail bed.
• Nail separation (onycholysis) — the nail plate lifts away from the nail bed, often starting at the free edge and progressing proximally.
• Distorted shape — in severe or long-standing cases, the nail plate becomes frankly deformed, with irregular ridging and shape distortion.

Causes and Risk Factors

Fungal organisms that cause onychomycosis are ubiquitous in the environment. Risk factors for developing nail infection include:

• Older age — cumulative exposure and reduced peripheral circulation increase susceptibility with age
• Diabetes — impaired immune response and circulation increase fungal infection risk significantly
• Tinea pedis (athlete’s foot) — untreated athlete’s foot frequently spreads to the nails; treat both conditions simultaneously
• Nail trauma — repeated microtrauma (tight shoes, running) creates entry points for fungal invasion
• Warm, moist environments — public pools, locker rooms, communal showers are transmission hotspots
• Occlusive footwear — boots, rubber shoes, and non-breathable footwear create humid microenvironments that favor fungal growth
• Immunosuppression — HIV, organ transplant recipients, biologic therapy users, and patients on high-dose corticosteroids

Confirming the Diagnosis Before Treatment

This is perhaps the most important and most overlooked step. Not all abnormal-looking nails are fungal. Nail psoriasis, trauma-related nail changes, lichen planus, and keratin granulations from nail polish can all look like onychomycosis. Starting oral antifungal therapy without confirming the diagnosis exposes the patient to a 12-week course of medication with potential side effects for no benefit.

In our practice, we confirm with one of two methods before prescribing oral antifungals:

• Nail clipping culture — a sample of the affected nail (free edge plus subungual debris) is sent to the lab in a dermatophyte test medium (DTM) culture. Takes 4–6 weeks but identifies the specific organism and confirms fungal vs non-fungal etiology. Gold standard.
• PAS staining — the nail clipping is processed histologically and stained with periodic acid-Schiff (PAS), which stains fungal cell walls. Results in 1–2 weeks; highly sensitive but doesn’t identify the specific organism. Suitable for most clinical decisions.

Treatment Options Compared

Treatment	Cure Rate	Duration	Notes
Terbinafine (Lamisil) oral	70–80%	12 weeks	First-line; liver function monitoring
Itraconazole oral (pulse)	55–65%	3 monthly pulses	More drug interactions than terbinafine
Efinaconazole (Jublia) topical	30–40%	48 weeks	Rx only; excellent tolerability
Ciclopirox (Penlac) topical	20–35%	48 weeks	Rx only; requires diligent daily application
Tavaborole (Kerydin) topical	25–35%	48 weeks	Rx only; oxaborole antifungal
Laser therapy	30–60%	3–4 sessions	Emerging evidence; no FDA “cure” claim
OTC antifungal polish	<10%	Months	Undecylenic acid; minimal clinical evidence

In our clinic, for healthy patients with confirmed dermatophyte onychomycosis, oral terbinafine is the first-line recommendation. The 12-week course has a 70–80% mycological cure rate — significantly better than any topical option. We obtain a baseline liver function test before starting and typically don’t re-check unless the patient has symptoms or pre-existing liver disease. Terbinafine is generally very well tolerated.

For patients who cannot take oral antifungals (liver disease, drug interactions, personal preference), prescription topical efinaconazole is the preferred option. Topicals work best for mild-to-moderate infections and should be continued for the full 48-week course consistently.

Over-the-Counter Remedies: What Actually Works?

The OTC market for toenail fungus products is large and largely ineffective for established infections. Here’s the honest assessment:

• OTC antifungal nail polishes (undecylenic acid) — the most commonly purchased category. Minimal clinical evidence for cure. May suppress surface fungal growth but cannot penetrate the nail plate adequately for eradication of deep infection. Appropriate for very superficial WSO only.
• Vicks VapoRub (thymol + camphor) — has antifungal properties in vitro; one small clinical study showed modest benefit. Not FDA-approved for onychomycosis but frequently attempted as a home remedy. We discuss it honestly with patients — worth a trial if they’re not ready for prescription treatment, but expectations should be realistic. See our detailed Vicks analysis.
• Tea tree oil — has in vitro antifungal activity. One small RCT showed results similar to clotrimazole cream. Insufficient evidence for standalone use in established onychomycosis.
• Listerine foot soaks — thymol and eucalyptol have antifungal properties, but soaking the nail in Listerine has no clinical evidence for cure. See our detailed Listerine analysis.

Prevention and Recurrence

Onychomycosis recurrence after successful treatment is a significant clinical challenge — recurrence rates of 20–25% within 1–3 years are reported. Prevention strategies:

• Treat concurrent tinea pedis — if you also have athlete’s foot, treat it simultaneously with oral terbinafine or a topical antifungal. Reinfection from athlete’s foot is a major source of nail recurrence.
• Protective footwear in public areas — always wear sandals or pool shoes in locker rooms, public pools, gym showers, and nail salons.
• Antifungal foot powder or spray — apply to shoes and feet after treatment completion. Reduces environmental fungal burden in footwear.
• Treat and discard old footwear — spraying the inside of shoes with antifungal spray (Lysol, tolnaftate spray) helps; severely contaminated shoes should be replaced.
• Keep nails short and dry — trim nails straight across, file the surface to reduce thickness, and dry feet thoroughly after bathing (including between toes).
• Prophylactic monthly topical antifungal — after successful treatment, applying efinaconazole or ciclopirox once weekly to the previously affected nails is supported by data as a recurrence-reduction strategy.

Frequently Asked Questions

How long does toenail fungus treatment take?

Oral terbinafine is taken for 12 weeks — the drug reaches therapeutic levels in the nail and remains active for months after stopping. However, the nail takes an additional 9–18 months to grow out completely clear after mycological cure, because toenails grow slowly (approximately 1.5 mm/month). The treatment ends at 12 weeks; the visible result isn’t obvious until the cleared nail has grown out over 12–18 months. Patients often assume treatment failed because the nail still looks abnormal at 3 months — this is normal.

Is toenail fungus contagious?

Yes — onychomycosis can spread from person to person through direct contact with infected nail or skin, or through contaminated surfaces (floors, nail salon equipment). It can also spread from one foot to the other, or from feet to fingernails via hand-to-foot contact. Using separate nail clippers for infected nails, wearing sandals in communal areas, and treating household members who have signs of tinea pedis or nail infection reduces transmission.

The Bottom Line

Toenail fungus is treatable — but requires a confirmed diagnosis and the right treatment approach. Oral terbinafine remains the most effective option for confirmed dermatophyte infection. Don’t spend months on OTC polishes for a culture-confirmed fungal infection that requires prescription therapy. If you have diabetes, are immunocompromised, or have been struggling with nail fungus for more than 6 months, a podiatric evaluation is the efficient path to clearing your nails.

Sources

1. Gupta AK, Stec N, Summerbell RC, et al. “Onychomycosis: a review.” Journal of the European Academy of Dermatology and Venereology. 2020;34(9):1972-1990.
2. Kreijkamp-Kaspers S, Hawke K, Guo L, et al. “Oral antifungal medication for toenail onychomycosis.” Cochrane Database of Systematic Reviews. 2017;7:CD010031.
3. Elewski BE, Rich P, Pollak R, et al. “Efinaconazole 10% solution in the treatment of toenail onychomycosis: two phase III multicenter, randomized, double-blind studies.” Journal of the American Academy of Dermatology. 2013;68(4):600-608.