Terbinafine: what it treats, how to take it, side effects
Last updated September 3, 2026.
Terbinafine is an antifungal tablet that cures nail fungus from the inside out. You may know it as Lamisil. It is the standard oral treatment for infected fingernails and toenails - and it demands patience, because the course runs 6 to 12 weeks and the nail keeps improving for months after the last pill.
What does it treat?
Onychomycosis - fungal infection of the toenail or fingernail caused by dermatophytes. The label asks for lab confirmation (a KOH test, culture, or nail biopsy) before treatment starts, because thick discolored nails are not always fungus. Doctors also sometimes prescribe terbinafine for ringworm of the skin and jock itch, per MedlinePlus.
How do you take it?
One 250 mg tablet once a day, with or without food: 6 weeks for fingernails, 12 weeks for toenails. Granules, used for children, get sprinkled on a spoonful of soft food like pudding or mashed potatoes (not applesauce) and swallowed without chewing. Here is the part people find strange: the fungus may not be fully gone when the pills run out, and the nail will not look normal right away. A healthy nail has to grow in, so the full result shows up months after you finish. Missed tablet dose: take it when you remember unless the next one is less than 4 hours away; never double up.
Side effects to know
- Common: diarrhea, indigestion, itching, headache, changes in sleep, and mood changes - feeling sad, worthless, restless, or losing interest in daily activities.
- Serious: liver problems lead (nausea, loss of appetite, vomiting, extreme tiredness, pain in the upper right stomach, dark urine, pale stools, yellowing skin or eyes); the label warns that rare cases of liver failure, some leading to death or liver transplant, have occurred in people with and without pre-existing liver disease. Also serious: severe or worsening skin rash, peeling or blistering skin, fever with sore throat or other signs of infection, unexplained bleeding or bruising, blood in the urine, chest pain, fast or irregular heartbeat, and allergic reactions with facial swelling or trouble breathing. One distinct effect: terbinafine can change or erase your sense of taste or smell - sometimes briefly, sometimes for a long time, sometimes permanently. Report it to your doctor if it happens.
Who should not take it?
If you have or have ever had liver disease, MedlinePlus says your doctor will probably tell you not to take terbinafine. Tell your doctor about HIV or AIDS, a weakened immune system, lupus, and kidney disease. Cimetidine (Tagamet) interacts with it. The drug makes skin more sun-sensitive, so use sunscreen and skip tanning beds. Do not breastfeed while taking it, and call your doctor if you become pregnant. Your doctor will order lab tests before you start and during treatment - those are the early-warning system for the liver risk, so keep the appointments.
When is it an emergency?
Call 911 for trouble breathing or swallowing or swelling of the face, throat, or tongue. Yellowing skin or eyes, dark urine, pale stools, or pain in the upper right stomach mean calling your doctor immediately - do not wait for the next scheduled pill. Same for a severe or spreading rash. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Why does terbinafine take so long to work on toenails?
Because the nail, not the pill, sets the schedule. The drug kills the fungus - 12 weeks for toenails, 6 for fingernails - but the damaged nail has to be replaced by new growth. MedlinePlus says the fungus may not be completely cured until a few months after you finish, because it takes time for a healthy nail to grow in. Judge results at the finish line, not at week 6.
What is the deal with the liver warning?
The label warns that rare cases of liver failure, some leading to death or liver transplant, have occurred with terbinafine tablets - in people with and without pre-existing liver disease. That is why MedlinePlus says a history of liver disease usually rules the drug out, why your doctor runs lab tests before and during treatment, and why nausea with dark urine, pale stools, or yellowing skin means an immediate call, not a wait-and-see.
Can terbinafine really change my sense of taste?
Yes. MedlinePlus notes terbinafine may cause a loss or change in taste or smell, which can bring decreased appetite, weight loss, and anxious or depressed feelings with it. The change may improve after you stop, may last a long time, or may be permanent. If you notice it, call your doctor - early reporting matters.
Can I drink alcohol while taking terbinafine?
Neither the FDA label nor MedlinePlus lists a specific alcohol interaction. Given that the drug carries a rare but real risk of liver injury, many clinicians suggest avoiding alcohol for the course - ask your doctor for advice on your situation.
Why do I need blood tests for a toenail pill?
Because of the liver risk. MedlinePlus says your doctor will order certain lab tests before you begin and during your treatment. Those tests are how a silent liver problem gets caught before it becomes a serious one, so skipping them defeats the safety net.
Does terbinafine interact with anything common?
MedlinePlus flags the over-the-counter heartburn drug cimetidine (Tagamet) as an interaction - do not start it while taking terbinafine without discussing with your healthcare provider. As always, hand your pharmacist the full list of what you take, including supplements.