Fluconazole blocks the enzyme fungi use to make ergosterol, the fatty substance that holds their outer membrane together. The membrane becomes leaky and the fungus stops growing. It is the same family of chemistry as clotrimazole and ketoconazole, but built to be swallowed rather than rubbed on.
What makes it useful is where it goes. It dissolves in water rather than fat, so it spreads into body fluids well and stays in the body for a long time — which is why one dose can last a week. That also means it does not concentrate in oily skin or in the nail plate the way itraconazole does, and it is one reason it performs less well for nails and thick foot skin. Its strength is against yeasts: Candida especially, and Malassezia. Against dermatophytes, the ringworm family, it works, but less reliably than terbinafine or itraconazole. It is absorbed regardless of food or stomach acid, which makes it far simpler to take than itraconazole.
Warfarin. Fluconazole raises the INR and can cause serious bleeding. If you are on warfarin, this needs to be flagged before the first dose and your INR checked.
Sulfonylurea diabetes tablets such as glipizide, glyburide and glimepiride. Fluconazole raises their levels and can cause low blood sugar.
Phenytoin, and some other epilepsy medicines. Levels rise and need watching.
Statins, particularly simvastatin and atorvastatin. Higher levels raise the risk of muscle damage.
Medicines that affect heart rhythm. Fluconazole can lengthen the QT interval, and combining it with other QT-lengthening drugs — some antipsychotics, ondansetron, citalopram, certain antibiotics — raises the risk of a dangerous rhythm. It should not be taken with pimozide, cisapride or quinidine at all.
Alcohol in any quantity that stresses the liver. There is no direct reaction, but both are handled by the liver.
Pregnancy. This is the part that is genuinely load-bearing. High doses of fluconazole (400 to 800 mg daily) taken through the first trimester have caused a recognised pattern of birth defects, and that use is contraindicated. A single low dose of 150 mg is a different situation, and the evidence is less clear — some studies have found a small increase in miscarriage, others have not. The practical position is that fluconazole is generally avoided in pregnancy when a topical treatment would do, and topical clotrimazole is the standard alternative for thrush. Discuss it rather than assuming either way.
A single dose needs no monitoring at all.
For courses of several weeks or longer, many prescribers check liver function before starting and occasionally during treatment. This is more of a routine precaution than a response to a common problem — significant liver injury from fluconazole is rare.
If you take warfarin, your INR should be checked shortly after starting and again after finishing. This is the single most important piece of monitoring on this page.
If you take a sulfonylurea for diabetes, watch your blood sugars more closely during the course.
An ECG is sometimes done before starting in people with known heart rhythm problems or on other QT-lengthening medicines.
Vaginal thrush
Itching usually eases within 24 hours and the infection clears over two to three days from a single dose.
Skin folds and Candida on the skin
Redness and soreness improve within the first week. Two to four weekly doses are typical.
Tinea versicolor and fungal acne
The itch and any new spots settle in one to two weeks. The pale or dark patches left by tinea versicolor take much longer — often two to six months to even out, and they need some sun exposure to repigment. That is not the drug failing; the fungus is gone and the pigment is catching up.
Ringworm and athlete's foot
Two to six weekly doses. Slower and less certain than terbinafine, which is why it is not the first pick.
Nails
Six to twelve months of weekly dosing, and the nail still takes another six to twelve months to grow out. Cure rates are lower than terbinafine or itraconazole. It is a reasonable option only when the other two are unsuitable.
There is no taper and no withdrawal. You stop when the course ends.
Whether it comes back depends entirely on what was treated. A one-off bout of thrush usually does not return. Tinea versicolor returns in most people within a year or two — typically the next warm season — because the yeast is a normal resident of the skin. Fungal acne behaves the same way. For both, a ketoconazole or selenium sulfide wash used once or twice a week is the standard way to hold them off, and stopping that maintenance is usually why people are back.
Candida in a skin fold returns whenever the fold gets warm and damp again, no matter how thoroughly the yeast was cleared. Drying and barrier care prevent that; tablets do not.
Some people are prescribed repeated weekly fluconazole for recurrent thrush over months. That is a deliberate maintenance plan rather than an ordinary course, and stopping it is done under guidance.