For toenail fungus, the standard course of oral terbinafine is 12 weeks (about 3 months). For fingernail fungus, it’s 6 weeks. These are the FDA-approved durations for a once-daily 250 mg tablet, and most prescriptions follow this schedule exactly.
What catches many people off guard is that even after finishing the medication, the nail won’t look normal right away. The drug eliminates the fungus, but the damaged nail has to physically grow out and be replaced by healthy nail. That process can take 12 to 18 months for toenails, which grow far more slowly than fingernails.
Toenails vs. Fingernails
The difference in treatment length comes down to how deep the drug needs to penetrate and how quickly each type of nail grows. Toenails are thicker, grow slower, and sit in an environment (shoes, socks) that’s friendlier to fungus. That’s why they need double the treatment time compared to fingernails.
- Toenail fungus: 250 mg once daily for 12 weeks
- Fingernail fungus: 250 mg once daily for 6 weeks
Your prescriber may occasionally adjust this timeline based on the severity of the infection or how many nails are affected, but the 6-week and 12-week courses are the standard benchmarks set by the FDA.
Why the Nail Still Looks Bad After Treatment
This is the part that frustrates people most. You finish your full course of terbinafine, and the nail still looks thick, discolored, or crumbly. That doesn’t mean the treatment failed. Terbinafine works by accumulating in the nail tissue and killing the fungus there, but it can’t repair nail that’s already damaged. The infected portion of the nail simply has to grow out on its own.
Fingernails typically replace themselves in about 6 months. Toenails are much slower, taking 12 to 18 months to fully grow out. So if you finish your 12-week terbinafine course in March, you might not see a completely clear toenail until the following winter or even spring. The key sign that treatment is working is healthy, normal-looking nail growing in at the base, gradually pushing the damaged nail forward.
Why You Shouldn’t Stop Early
It’s tempting to quit once you start seeing improvement, but cutting the course short is one of the most common reasons nail fungus comes back. Terbinafine needs enough time to build up sufficient concentrations in the nail plate to fully eliminate the infection. Stopping at week 8 instead of week 12, for example, may leave enough surviving fungus to reinfect the nail as it grows.
If you miss a dose, take it as soon as you remember and continue your normal schedule. Don’t double up. Consistency matters more than perfection here.
Side Effects to Watch For
Most people tolerate terbinafine well, but there are a few side effects worth knowing about. The most common ones are mild: headache, diarrhea, rash, and digestive discomfort.
The more distinctive side effect is taste disturbance, which affects roughly 3% of people taking the drug. Food may taste metallic, muted, or just “off.” For most people this resolves within several weeks of stopping the medication. In rare cases, though, altered taste can persist for more than a year or become permanent. If you notice a significant change in your sense of taste while taking terbinafine, let your prescriber know.
Liver Safety During Treatment
Terbinafine is processed through the liver, and in rare instances it can cause liver irritation. The FDA recommends a blood test to check liver enzymes before you start the medication. In the 1990s, repeat testing during treatment was also recommended, but that guideline was removed in 2001. Current practice is to check liver function at baseline and then only retest if you develop symptoms like unusual fatigue, dark urine, pale stools, nausea, or yellowing of the skin or eyes.
People with a history of liver disease will typically need to discuss alternative treatments with their prescriber, since terbinafine may not be the safest choice in that situation.
What If the Fungus Comes Back
Even after a successful course, nail fungus has a meaningful recurrence rate. The same warm, moist conditions that caused the original infection can invite it back. A few practical steps help reduce your odds: keep your feet dry, rotate shoes to let them air out, wear moisture-wicking socks, and use antifungal powder or spray as a preventive measure in shoes.
If the infection does return, a second course of terbinafine is sometimes prescribed. Your prescriber may also consider combination approaches, such as pairing oral medication with a topical antifungal applied directly to the nail. The decision depends on how severe the recurrence is and how you tolerated the first round of treatment.