Fungi build their outer membrane from ergosterol. Terbinafine blocks squalene epoxidase, an enzyme early in that process. The fungus runs short of ergosterol and, at the same time, a substance called squalene piles up inside it until it is toxic. That combination kills the fungus rather than just stopping it dividing, which is why a week of terbinafine does what four weeks of clotrimazole does.
Coverage is the thing to understand about this drug. Terbinafine is excellent against dermatophytes — ringworm, jock itch, athlete's foot, nail fungus. It is weak against Candida, the yeast that causes most skin-fold rashes, so it is the wrong choice in the groin crease, under a breast or under a belly fold. Applied to the skin it does work on Malassezia, the yeast in tinea versicolor, because the concentration on the surface is high. Terbinafine tablets do not, which is a genuinely important difference — the cream treats tinea versicolor and the tablets do not.
Steroid creams on the same patch unless a doctor has told you to. Steroid settles the itch and lets the fungus spread outward until the rash no longer looks like ringworm at all.
Using it on a moist skin-fold rash and expecting results. Terbinafine does not cover Candida.
Eyes, mouth and vagina. External skin only.
Airtight dressings over the treated area.
Untreated shoes. Powder or spray the insides while you treat your feet, and alternate pairs so each dries fully.
No interactions with tablets, alcohol or sunlight. Almost none of the cream reaches the bloodstream, so the liver and drug-interaction cautions that apply to terbinafine tablets do not apply here.
Days two to four
Itch settles, often noticeably faster than with an azole cream.
Days five to ten
Redness and scaling clear on the body. Between the toes, the white soggy skin dries and firms up.
Weeks two to four
The thick, dry, scaly kind of athlete's foot on the sole takes longest, and sometimes still needs terbinafine tablets. The cream keeps working in the skin for one to two weeks after your last application, so improvement continues after you stop.
No change at two weeks
Stop and rethink. Either it is not a dermatophyte — Candida is the usual answer in a fold — or it is not fungal.
The marks afterwards
Lighter or darker patches can stay for weeks to months where the rash was. This is more noticeable on brown and Black skin and it is pigment, not infection. In tinea versicolor the pale patches often take months to repigment, and they need some sun exposure to even out; that is not treatment failure.
You stop at the end of the course and the drug carries on working in the skin for another week or two, so improvement usually continues.
Ringworm and jock itch treated fully do not normally return. Athlete's foot does return for a lot of people, and the reason is almost never the cream — it is untreated shoes, communal showers and feet that stay damp. An antifungal powder used a few times a week is the maintenance step.
If you were using it on the skin around an infected toenail, stopping means the skin gets reinfected from the nail. That is the argument for treating the nail itself rather than just the skin around it.
Tinea versicolor comes back in most people, typically in warm weather. A ketoconazole or selenium sulfide wash used occasionally is the usual way to hold it off.