Fungi build their outer membrane from ergosterol. Terbinafine blocks squalene epoxidase, an enzyme at the start of that process. The fungus runs out of ergosterol and, at the same time, squalene accumulates inside it until it is toxic. The fungus dies rather than simply stopping — the difference between killing and stalling is why terbinafine courses are shorter than azole courses.
Taken by mouth, terbinafine collects in exactly the places creams cannot reach: the fat under the skin, the nail bed and the growing hair. It also lingers there for months after the last tablet, which is why a nail keeps improving long after the course is over. Its coverage is narrow but deep. It is outstanding against dermatophytes — the fungi behind ringworm, athlete's foot, jock itch, scalp ringworm and most nail infections. It is weak against Candida and against Malassezia, which means it does not treat tinea versicolor, fungal acne or a yeasty skin fold. If you have been prescribed terbinafine tablets for one of those, it is worth asking about, because the cream works on tinea versicolor and the tablets do not.
Certain other medicines. Terbinafine blocks a liver enzyme called CYP2D6, which raises the levels of a number of common drugs. The ones that matter most are tricyclic antidepressants such as amitriptyline and nortriptyline, some SSRIs including paroxetine and fluoxetine, beta blockers such as metoprolol, some heart rhythm drugs including flecainide and propafenone, and tamoxifen — where terbinafine can reduce how well the tamoxifen works. Rifampin lowers terbinafine levels; cimetidine raises them. Give your prescriber a full list of everything you take, including anything bought over the counter.
Heavy alcohol. There is no dangerous reaction to a normal drink, but terbinafine is processed by the liver and heavy drinking during a three-month course adds risk that is easy to avoid.
Caffeine, mildly. Terbinafine slows how quickly you clear it, so coffee may hit harder than usual. This is a nuisance, not a hazard.
Pregnancy and breastfeeding. Terbinafine passes into breast milk. Because treating a nail is not urgent, both are usually reasons to wait rather than to accept an unknown.
Ignoring new symptoms. Dark urine, yellowing of the eyes, persistent nausea, unusual tiredness or a spreading rash all need a call, not a wait-and-see.
A liver blood test (ALT and AST) before starting is standard practice for any course longer than a couple of weeks, and is expected before a nail course. Some prescribers also check a full blood count.
For courses of six weeks or more, many dermatologists repeat the liver tests at around four to six weeks. Practice varies — some rely on symptom reporting instead, because liver injury from terbinafine tends to appear suddenly rather than creeping up on a blood test. Both approaches are defensible; what is not defensible is starting a three-month course without a baseline.
What is actually being watched for is a rise in liver enzymes, a drop in white cells, and above all the symptoms: nausea that will not settle, dark urine, yellow eyes, itching all over. Your reporting matters more than the schedule of blood draws.
For nails, a clipping sent for testing before starting is not monitoring exactly, but it belongs here — it is the step that stops people taking three months of tablets for a nail that was never infected.
Skin infections
Two weeks of tablets clears most athlete's foot and body ringworm, with visible improvement within the first week. Because the drug stays in the skin for weeks afterwards, it keeps working after the last tablet.
Scalp ringworm
Four to six weeks. Scaling and itch settle first; hair regrows over the following two to three months.
Nails — the part people are not warned about
Nothing visible happens for months. The tablets stop the fungus, but the damaged nail has to grow out, and a toenail grows about 1 mm a month. A new, clear nail appears at the base first. Expect nine to twelve months for a big toenail to look normal, and six months for a fingernail, even though the tablets stop at twelve weeks. Judging the treatment at the end of the course is judging it far too early.
What success actually looks like
In trials, the fungus is cleared in about seven out of ten people and the nail looks completely normal in about four or five out of ten. Many others end up with a nail that is better but not perfect. That is a realistic expectation, not a disappointing one — no other treatment does better.
If the nail is unchanged at nine months
Go back. It may be a non-dermatophyte mould, which terbinafine covers poorly, or the nail may never have been fungal.
You simply stop at the end of the course. There is no taper and no withdrawal.
Terbinafine stays in the nail and skin for months, so the treatment carries on working after the last tablet — which is why nails keep improving through the following year.
Relapse is the real issue with nails. Somewhere between a fifth and a quarter of people see the infection return within a few years. The usual routes are untreated athlete's foot on the surrounding skin, shared shower floors, and shoes that were never dealt with. Treating the skin with a cream during and after the course, using an antifungal powder in shoes, and drying between the toes all reduce that meaningfully.
If taste changed during treatment, it usually starts recovering within a few weeks of stopping, though it can take several months.
A second course is possible if the infection returns, but it is worth confirming the diagnosis again first rather than repeating three months of tablets on assumption.