Terbinafine is an antifungal medication used to treat fungal infections of the skin and nails. It comes in two main forms: a topical cream or spray for skin infections like athlete’s foot, jock itch, and ringworm, and an oral tablet for more stubborn fungal nail infections. It’s one of the most commonly prescribed antifungals worldwide, and it works by targeting an enzyme that fungi need to build their cell walls.
How Terbinafine Kills Fungi
Fungi rely on a substance called ergosterol to keep their cell membranes intact, similar to how human cells use cholesterol. Terbinafine blocks the enzyme responsible for producing ergosterol. Without it, the fungal cell membrane weakens and the cell dies. At the same time, a fatty compound called squalene builds up inside the fungal cell because it can no longer be converted into ergosterol, which adds to the toxic effect.
One reason terbinafine is well tolerated in humans is selectivity. It binds to the fungal version of this enzyme roughly 2,000 times more tightly than to the human version. It also doesn’t interfere with the family of liver enzymes that many other antifungals disrupt, which reduces certain types of drug interactions (though it has its own interaction profile, covered below).
Fungal Nail Infections
Oral terbinafine is the go-to treatment for onychomycosis, the medical term for fungal nail infections. These infections cause nails to thicken, discolor, and crumble at the edges. They’re notoriously difficult to treat because the fungus lives beneath the nail plate, where topical treatments struggle to reach.
The typical course is one tablet daily for 6 weeks for fingernail infections and 12 weeks for toenail infections. Toenails take longer because they grow more slowly, and the medication needs to remain in the nail bed long enough to clear the infection as the nail grows out. Even after finishing the course, it can take several months for the nail to look fully normal, since you’re waiting for a healthy nail to replace the damaged one. This doesn’t mean the treatment failed.
Skin Infections Treated With Topical Terbinafine
The cream and spray forms treat a range of common fungal skin infections. Treatment durations are much shorter than for nail infections, typically one to four weeks depending on the condition.
- Athlete’s foot (tinea pedis): Applied twice daily for one to four weeks. Infections between the toes often clear faster than infections on the sole of the foot, which typically need at least two weeks.
- Jock itch (tinea cruris): Applied once or twice daily for one to four weeks.
- Ringworm (tinea corporis): Applied once or twice daily for one to four weeks. Despite the name, ringworm has nothing to do with worms. It’s a fungal infection that creates a ring-shaped rash.
- Tinea versicolor: Sometimes called “sun fungus,” this causes light or dark patches on the skin. Treated with topical terbinafine once or twice daily for about two weeks.
- Yeast infections of the skin (cutaneous candidiasis): Applied once or twice daily for one to two weeks.
The spray formulation tends to have shorter treatment courses than the cream, often around seven days. Both are available over the counter for athlete’s foot, jock itch, and ringworm, while oral terbinafine requires a prescription.
Common Side Effects
Topical terbinafine rarely causes problems beyond mild skin irritation at the application site. Oral terbinafine carries more potential side effects because the drug circulates throughout the body.
The most common complaints with oral terbinafine are gastrointestinal: nausea, diarrhea, and stomach discomfort. These are usually mild and resolve on their own. A more unusual side effect is taste disturbance or complete loss of taste, which occurs in roughly 0.6% to 2.8% of people taking the oral form. Taste typically returns within several weeks of stopping the medication, though in rare cases it can take longer.
Liver problems are an uncommon but serious risk. Your doctor will likely check liver function with a blood test before starting treatment and may recheck during a longer course. Symptoms to watch for include unusual fatigue, dark urine, or yellowing of the skin or eyes.
Drug Interactions
Oral terbinafine strongly inhibits a specific liver enzyme called CYP2D6, which the body uses to break down a long list of medications. If you take terbinafine alongside drugs processed by this enzyme, those drugs can build up to higher-than-normal levels in your body.
The most clinically relevant interactions involve several common drug classes: certain antidepressants (both SSRIs and tricyclics), some beta-blockers used for blood pressure and heart rate, certain heart rhythm medications, and some antipsychotics. In one study, terbinafine decreased the clearance of a tricyclic antidepressant by 82%, a dramatic increase in drug exposure. It can also reduce the effectiveness of codeine, because codeine relies on the same enzyme to be converted into its active pain-relieving form. For people taking tamoxifen for breast cancer, terbinafine may reduce the drug’s active form in the body, potentially weakening its effect.
Before starting oral terbinafine, make sure your prescriber knows every medication you take, including over-the-counter drugs and supplements.
Kidney and Liver Considerations
People with significant kidney impairment (roughly less than half of normal kidney function) or liver cirrhosis clear terbinafine from the body about 50% more slowly than people with healthy organs. This means the drug hangs around longer and reaches higher levels. Oral terbinafine is generally not recommended for people with active or chronic liver disease, and kidney impairment may require dose adjustments or closer monitoring. Topical terbinafine isn’t affected by these concerns because very little of it enters the bloodstream.
What to Expect During Treatment
For skin infections, you’ll likely notice improvement within the first week or two. The key is finishing the full course even after symptoms disappear, because the fungus can still be alive beneath apparently healthy skin.
For nail infections, patience matters. You probably won’t see visible improvement for the first month or two. The medication deposits into the nail bed and kills the fungus there, but the damaged nail above it doesn’t repair itself. It grows out gradually and gets trimmed away. A new, healthy fingernail takes about three to six months to fully grow in. Toenails can take nine to twelve months. The fact that the treatment course ends long before the nail looks normal is a common source of frustration, but it doesn’t indicate treatment failure. The full results aren’t visible until the old nail has been completely replaced.