Itraconazole blocks the enzyme fungi need to make ergosterol, the fat that holds their outer membrane together. The membrane fails and the fungus cannot grow. That is the shared azole mechanism, but itraconazole covers an unusually wide range with it: dermatophytes, Candida, Malassezia, and several moulds that terbinafine misses.
Where it goes in the body explains how it is dosed. Itraconazole dissolves in fat, so it concentrates in skin, in the oil glands and in the nail plate, and it stays there for weeks after the last capsule. That is why nails are treated in pulses — a week of capsules, three weeks off, repeated — and why the drug carries on working long after you stop. The same fat-loving chemistry is why food matters: the capsules need a full meal and an acidic stomach to dissolve and be absorbed. Taken on an empty stomach, or alongside an antacid, a large part of the dose simply does not get in.
This is the most important section on this page. Itraconazole strongly blocks a liver enzyme called CYP3A4, which handles a very large share of common medicines. Blocking it can push other drugs to dangerous levels.
Never combine with: simvastatin and lovastatin (risk of severe muscle breakdown), pimozide, quinidine, dofetilide, cisapride, ergot medicines used for migraine, oral midazolam and triazolam, and ranolazine. This is not the full list — your prescriber must check yours.
Use only with specific advice: other statins, some calcium channel blockers, ticagrelor and other blood thinners, warfarin, colchicine, methadone, certain HIV and cancer medicines, some antipsychotics, and cyclosporine or tacrolimus.
Medicines that make itraconazole fail: rifampin, phenytoin, carbamazepine and St John's wort all speed up its breakdown so much that the treatment may not work at all.
Acid reducers. Proton pump inhibitors, H2 blockers and antacids reduce absorption of the capsules. This is a practical failure point, not a safety one, but it is a common reason a course does not work.
Heart failure. Itraconazole reduces how forcefully the heart contracts and can cause or worsen heart failure. It is contraindicated for nail treatment in anyone with heart failure or a history of it.
Pregnancy and contraception. Itraconazole is contraindicated for nail treatment in pregnancy and in women who may become pregnant unless they are using effective contraception, which is advised throughout the course and for two months after the last dose. It also has interactions with some hormonal contraceptives, so that is worth raising directly.
Grapefruit juice, which interferes with the same enzyme, and heavy alcohol, which adds liver strain.
A full review of every medicine you take, including over-the-counter and herbal products, before the first capsule. That is the single most important check with this drug.
Liver function blood tests before starting, and repeated during courses lasting longer than about a month. Practice varies for short skin courses; a baseline is usual before nail treatment.
An assessment of heart function — usually just a history and examination — before treating nails, because heart failure is a contraindication.
During treatment, what is being watched for is ankle swelling and breathlessness, liver symptoms, and any new medicine being started by another prescriber. If someone adds a statin or an antibiotic while you are on itraconazole, that needs checking.
Potassium and blood pressure are sometimes checked on longer courses.
Tinea versicolor
One five to seven day course usually clears the yeast. The rash stops spreading almost immediately. The pale or brown patches take two to six months to even out and need some sun to repigment — that stage is not treated by more tablets.
Fungal acne
Often the fastest response of anything used for it. Itch settles within days and the spots stop appearing within one to three weeks.
Ringworm and athlete's foot
Visible improvement within the first week; clearance over two to four weeks including the time after the capsules stop, because the drug stays in the skin.
Nails
Nothing visible for months. The pulses stop the fungus, but the damaged nail must grow out at about 1 mm a month. Expect nine to twelve months for a toenail to look normal after a three-pulse course, and around six months for a fingernail. Roughly six to seven in ten people clear the fungus, and fewer end up with a completely normal-looking nail. Judging it at the end of the third pulse is far too early.
Seborrheic dermatitis
Used rarely, and for control rather than cure. It comes back, and the shampoos are still the long-term plan.
You stop at the end of the course or the last pulse. There is no taper and no withdrawal.
Itraconazole stays in skin and nail for weeks after the last capsule, which is the whole point of pulse dosing — the treatment keeps working during the off weeks and for some time afterwards.
Recurrence depends on what was treated. Tinea versicolor comes back in most people within a year or two, usually in warm weather, and an oral course does not prevent that any better than a topical one does — the maintenance wash is what holds it off. Fungal acne behaves the same way. Nail infections return in roughly a fifth to a quarter of people over several years, usually because the athlete's foot on the surrounding skin was never dealt with.
If you were told to use contraception because of the pregnancy contraindication, that advice runs for two months after the last dose, not from the day you stop.