Famciclovir: what it treats, how to take it, side effects
Last updated September 3, 2026.
Famciclovir is an antiviral - it stops the herpes virus from spreading in the body. You may know it as Famvir. With herpes viruses, timing is the whole game: every schedule on this page starts at the first symptom.
What does it treat?
Shingles (herpes zoster), repeat cold sores and fever blisters, repeat outbreaks and suppression of genital herpes in people with normal immune systems, and returning herpes infections of skin and mucus membranes in people with HIV. It eases pain, burning, tingling, and itching, helps sores heal, and prevents new sores - but it does not cure herpes, and it may not stop transmission to others.
How do you take it?
Tablets with or without food, on a schedule that depends on the job: shingles - every 8 hours for 7 days, starting within 3 days of the rash; cold sores - a single dose at the earliest tingle, itch, or burn; genital herpes outbreaks - twice a day for one day, starting within 6 hours of the first symptom; suppression - twice a day for up to a year; herpes with HIV - twice a day for 7 days. Take it as soon as possible after symptoms appear - the windows are where the benefit lives. Exactly as directed, and finish the course. Missed dose: take it when you remember unless the next one is close; never double up.
Side effects to know
- Common: headache, nausea, vomiting, diarrhea or loose stools, gas, stomach pain, tiredness, rash, itching, and painful menstrual periods.
- Serious - call your doctor immediately: pain, burning, numbness, or tingling in the hands or feet - the nerve signal.
Who should not take it?
Flag kidney disease - the dose adjusts to kidney function - and weakened immunity beyond the listed uses. Galactose intolerance issues apply to the tablets. For genital herpes: the drug does not remove transmission risk - the protection conversation with partners stays. If you become pregnant, call your doctor.
When is it an emergency?
Shingles near the eye - rash on the forehead, nose, or around the eye - is same-day urgent care: vision is at stake. Face or throat swelling or trouble breathing is a 911 call. 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.
What a Pymander AI doctor consult looks like
Illustrative example, not a real member's messages.
Common questions
Why does everyone say to start it so fast?
The virus replicates first and hurts second - antivirals stop replication, so the benefit concentrates in the early window: within 3 days of the shingles rash, at the first tingle for cold sores, within 6 hours for a genital herpes outbreak. Miss the window and the drug has much less to stop. The practical move: if you get recurrent outbreaks, keep the medication at home so the first tingle and the first tablet are minutes apart, not a pharmacy trip apart.
Will it cure my herpes?
No - herpes viruses hide in nerve cells where no drug reaches them. What famciclovir does: shortens outbreaks, eases the pain and itching, helps sores heal faster, prevents new sores during an outbreak, and - taken daily as suppression - reduces how often outbreaks come. It also does not guarantee you will not transmit the virus: the partner-protection conversation stays even on suppression.
Why does shingles treatment run 7 days but a cold sore is one dose?
Different battles: shingles is a full nerve-root reactivation - the virus is actively multiplying for days, so the course runs every 8 hours for 7 days to cover the whole wave. A cold sore is a small, fast event caught at the tingle - one big dose at the earliest symptom covers it. The schedules are matched to the biology, which is why following yours exactly matters.
What is the shingles-and-the-eye warning?
Shingles on the forehead, nose, or around the eye can invade the eye itself - cornea damage, vision loss. Rash anywhere near the eye is same-day urgent care, antiviral or not. The other shingles long-game: postherpetic neuralgia - pain that outlasts the rash. Early antivirals lower that risk, and persistent pain after healing is a named, treatable problem: say it out loud at the follow-up.
Why did my doctor ask about my kidneys?
Famciclovir clears through the kidneys - impaired kidneys let it accumulate, so the dose and schedule adjust to kidney function. This is also why hydration matters during the course. New numbness or tingling in hands or feet on the drug is the call-immediately item - kidneys and nerves are the two watch fronts.
What happens if I miss a dose of famciclovir?
Take it as soon as you remember, unless it is almost time for the next one - then skip it and never double up. On the every-8-hours shingles schedule, alarms keep the antiviral level steady; on the one-day outbreak courses, a forgotten dose window matters more - call your pharmacist if you are unsure whether to continue the course.