Acyclovir: what it treats, how to take it, side effects
Last updated September 3, 2026.
Acyclovir is an antiviral - it stops herpes-family viruses from spreading in the body. You may know it as Zovirax. It treats shingles, genital herpes, and chickenpox, and the single biggest factor in how well it works is how fast you start it.
What does it treat?
Chickenpox, herpes zoster (shingles), and genital herpes. Two honest limits from MedlinePlus: acyclovir will not cure herpes infections, and it may not stop the spread of herpes to other people.
How do you take it?
With or without food, two to five times a day depending on the condition, at the same times every day - starting as soon as possible after symptoms begin. Speed matters: the label's shingles data shows treatment started within 72 hours of the rash works best, ideally inside 48. A typical shingles course is 800 mg five times a day; genital herpes runs 400 mg doses. Finish the whole prescription even when you feel better - stopping early can make the infection harder to treat. Missed dose: take it when you remember unless the next one is close; never double up.
Side effects to know
- Common: nausea, vomiting, diarrhea, and feeling unwell or tired.
- Serious - call your doctor immediately or get emergency care: allergic reactions (hives, rash, itching, trouble breathing or swallowing, facial or throat swelling, hoarseness); blistering or peeling skin; rash with fever and swollen glands; the kidney-blood cluster (pale skin, unusual bruising or bleeding, decreased urination, blood in the urine, swelling of the legs, feet, or ankles, yellowing of the skin or eyes, shortness of breath, unusual tiredness); and nervous-system effects - confusion, agitation, aggressive behavior, trouble speaking, numbness or tingling, hallucinations, uncontrollable shaking, loss of consciousness, or seizures.
Who should not take it?
Flag kidney disease, immune-system problems including HIV and AIDS, and any dehydration from recent illness - the kidneys clear this drug, and they need to be working and watered. For genital herpes: the virus can spread through sexual contact even with no blisters and even on acyclovir - that conversation, and whether partners should be treated, belongs with your doctor. If you become pregnant, call your doctor.
When is it an emergency?
Trouble breathing or swallowing, a seizure, or loss of consciousness is a 911 call. Blood in the urine, or confusion with hallucinations, means immediate care. 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 everyone say to start acyclovir immediately?
Because the clock is the treatment. MedlinePlus says to start as soon as possible after symptoms begin, and the label's shingles studies show the best results when treatment starts within 72 hours of the rash - ideally inside 48. The virus replicates fastest at the start; every day of delay is ground the drug cannot win back. Tingle, call, same day.
Can I still spread herpes while taking acyclovir?
Yes. MedlinePlus is direct: genital herpes can spread through sexual contact even without blisters or symptoms, and possibly even while taking acyclovir - and acyclovir does not cure the infection. The medication shortens and softens episodes; it does not make them contagious-proof. Talk with your doctor about reducing spread and whether partners should be treated.
Why do I need to drink extra water on acyclovir?
The kidneys clear it, and it can stress them. MedlinePlus flags dehydration and kidney disease as things to disclose before starting, and the serious-effects list includes decreased urination, blood in the urine, and ankle swelling - the kidney picture. Steady fluids through the course is the simple protection.
Will acyclovir cure my herpes?
No - MedlinePlus says it outright. Acyclovir stops the virus from spreading in the body during an outbreak: symptoms come and heal faster, and daily suppressive dosing can cut how often outbreaks happen. But the virus stays in the body, and that honesty matters for how you plan around it.
Can I drink alcohol while taking acyclovir?
Neither the label nor MedlinePlus gives acyclovir-specific alcohol language. Alcohol dehydrates - the opposite of what your kidneys need on this drug - and you are recovering from a viral infection. Light or none for the course is the practical call.
What happens if I miss a dose of acyclovir?
Take it as soon as you remember, unless it is almost time for the next one - then skip it and never double up. With up to five doses a day, phone alarms earn their keep: the frequent schedule exists to keep drug levels over the virus's replication rate, and gaps let it catch up.