Terbinafine (Lamisil): What it treats and how to use it safely

Terbinafine is a common antifungal you’ve probably heard of as Lamisil. It works by killing fungi that cause athlete’s foot, ringworm, jock itch, and fungal nail infections. You can get it as a cream or gel over the counter for skin infections, and as a prescription pill for stubborn skin or nail fungus. Below are straight answers about when to use it, how to take it, and what to watch for.

How to take terbinafine

Topical terbinafine (cream/gel) is usually used once or twice daily for 1–4 weeks depending on the infection. Follow package directions and keep treating for the full recommended time even after symptoms improve. For nail fungus, oral terbinafine is the standard option: the usual dose is 250 mg once daily. Treatment length varies—about 6 weeks for fingernail infections and 12 weeks for toenails. Nail improvements are slow; nails can take months to look normal as healthy nail grows in.

Before starting oral terbinafine your doctor should ask about liver disease and medicines you take. A baseline liver test (ALT/AST) is often recommended for people with risk factors. Don’t stop early just because the nail looks a bit better—stopping before the full course increases the chance the fungus returns.

Side effects, risks, and red flags

Most people tolerate topical terbinafine well — mild local irritation or itching is the common complaint. Oral terbinafine has more potential for side effects: taste change or loss (dysgeusia), nausea, headache, and rash are relatively common. Rare but serious problems include liver injury and blood disorders. Call a doctor right away if you notice yellowing of the skin or eyes, dark urine, persistent nausea, unusual bruising, or severe rash.

Terbinafine interacts with other drugs. It blocks CYP2D6, which can raise levels of medicines broken down by that enzyme. Important examples include some antidepressants, beta-blockers, and tamoxifen (where terbinafine can reduce tamoxifen’s effectiveness). Tell your prescriber about all medications, including supplements.

Oral terbinafine is usually avoided in people with active liver disease and is not recommended during pregnancy or breastfeeding unless a doctor says it’s necessary. Topical terbinafine is considered safer in those situations but check with your clinician first.

Simple steps help treatment work better: keep feet dry, rotate shoes, treat athlete’s foot early (it can reinfect nails), don’t share towels, and replace old socks. If topical treatment fails or the infection returns, see a healthcare provider for evaluation and possible oral therapy.

Got more questions about a specific case—like pregnancy, other meds, or a stubborn nail infection? Ask your pharmacist or doctor. Terbinafine is effective when used correctly, but a few precautions make a big difference in safety and success.