Aciclovir
1 medicine
Aciclovir is an antiviral that treats cold sores, genital herpes and shingles by blocking viral DNA replication. It is cleared by the kidneys, so you need good hydration and a lower dose if your kidney function is reduced.
Key facts
- Aciclovir (sold as Zovirax and other generics) is an antiviral used against herpes simplex and varicella-zoster virus infections: cold sores, genital herpes, chickenpox and shingles.
- It comes as tablets, a suspension, a cream for cold sores, and an intravenous form for severe infections. Starting it within 24 to 72 hours of symptoms works best.
- Aciclovir is cleared by the kidneys largely unchanged. Drink plenty of water while taking it, and your prescriber must lower the dose if your kidney function is reduced, to avoid crystals forming in the kidney tubules or confusion and tremor from drug buildup.
- Seek urgent care for reduced urination, confusion, or a rash that spreads or blisters.
What aciclovir treats
Aciclovir treats infections caused by herpes simplex virus (cold sores on the lips and face, genital herpes) and varicella-zoster virus (chickenpox and shingles). It can also be taken daily at a lower dose to suppress frequent genital herpes recurrences. It does not cure the virus or eliminate it from the body; it shortens outbreaks and lowers how often they happen.
How aciclovir works
Aciclovir is a guanosine analogue. Once inside a cell infected with herpes virus, a viral enzyme (thymidine kinase) converts it to its active triphosphate form far more efficiently than in uninfected cells. That active form blocks the viral DNA polymerase and stops the growing DNA chain, so the virus cannot copy itself. Healthy cells are largely spared because they do not have enough of the viral enzyme to activate the drug.
Before you take it
- Tell your prescriber about any kidney problems, dehydration, or other medicines that affect the kidneys; these change your dose.
- Drink water regularly during treatment, especially with the higher doses used for shingles or intravenous aciclovir.
- Aciclovir cream is for cold sores on the lips and face only; it is not for use inside the mouth, eyes, or genital area.
- If you are pregnant or breastfeeding, discuss the risks and benefits with your prescriber first.
Side effects
Common effects are nausea, headache, and diarrhoea. Cream can cause mild stinging or dryness at the application site.
Stop and seek urgent medical care for any of these:
- Little or no urine, or swelling of the legs or face.
- Confusion, agitation, hallucinations, tremor, or drowsiness.
- A severe skin reaction with blistering or peeling.
- Signs of an allergic reaction: swelling of the face or throat, or difficulty breathing.
Safety essentials
- Because aciclovir is renally cleared, staying well hydrated and matching the dose to your kidney function is the single most important safety step, particularly with intravenous treatment or in older adults. Poor hydration or an unadjusted dose risks crystal formation in the kidneys and reversible neurotoxicity.
- Do not use more cream or take more tablets than prescribed; higher doses do not clear an outbreak faster and raise the risk of side effects.
- Tell your prescriber about all other medicines, since some, including probenecid, can raise aciclovir levels further.
This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.