Aciclovir is an antiviral medicine for herpes viruses. It treats cold sores, genital herpes, shingles and chickenpox, and is used long term to prevent recurrent outbreaks. It comes as tablets, cream and an intravenous form. It works best started early — within the first day or two of symptoms.
What is Aciclovir?
Aciclovir (also spelled acyclovir) is available in South Africa in several forms:
| Product | Contains | Schedule |
|---|---|---|
| Acitop, Lovire, Zovirax, Zotrin | Aciclovir 5% cream | S1 / S4 |
| Aciclovir Sandoz, Cyclivex, Lovire | Aciclovir 200 mg tablet | S4 |
| Various | Aciclovir 400 mg, 800 mg tablets | S4 |
| Hospital preparations | Aciclovir 25 mg/ml infusion | S4 |
Being dispensed one brand rather than another is ordinary generic substitution.
Aciclovir works through an unusually elegant mechanism, and it explains why it is so well tolerated.
Aciclovir enters cells as an inactive compound. To become active it must be phosphorylated — and the enzyme that does the first step, thymidine kinase, is a viral enzyme. Only cells actually infected with a herpes virus have it.
So aciclovir is activated almost exclusively inside infected cells. Once active, it blocks the viral DNA polymerase and gets incorporated into the growing viral DNA chain, stopping it. Uninfected human cells barely activate it at all.
That selectivity is why aciclovir is remarkably safe for an antiviral, and why side effects are so few compared with medicines that act on all cells.
Two consequences matter practically:
- It only works on actively replicating virus. Herpes viruses lie dormant in nerve cells between outbreaks, and aciclovir cannot reach them there. It does not cure herpes or clear the virus from the body — it treats and suppresses outbreaks.
- Timing is critical. Because it acts on replication, starting within the first 24 to 72 hours of symptoms makes a substantial difference. Started late, the benefit is much smaller — which is why people with recurrent outbreaks are often given a supply to keep at home.
What is Aciclovir commonly used for?
- Cold sores (herpes labialis) — cream for mild outbreaks, tablets for frequent or severe ones.
- Genital herpes, both treating outbreaks and suppressing recurrences.
- Shingles (herpes zoster), where prompt treatment reduces severity and may reduce the risk of lasting nerve pain.
- Chickenpox, particularly in adults, adolescents or people with weakened immunity, where it is more severe.
- Suppressive therapy, taken daily long term to reduce the frequency of recurrent genital herpes or cold sores.
- Herpes simplex encephalitis, a brain infection — a medical emergency treated intravenously.
- Eye herpes infections, using specific eye preparations.
- Preventing herpes infections in people with weakened immunity, including after transplants or during chemotherapy.
It does not treat HIV, despite sitting in this antiviral category, and it does not work on the viruses that cause colds or flu.
On shingles specifically: treatment is most effective started within 72 hours of the rash appearing, and this is worth acting on. Shingles in the eye area, or with severe pain, needs same-day medical attention.
How Aciclovir fits into this medicine category
| Group | Examples | Used for |
|---|---|---|
| Anti-herpes antivirals | Aciclovir, valaciclovir, famciclovir | Herpes simplex, shingles, chickenpox |
| Antiretrovirals (NRTIs) | Tenofovir, lamivudine | HIV |
| Integrase inhibitors | Dolutegravir (Acriptega) | HIV |
| Anti-influenza | Oseltamivir | Influenza |
| Hepatitis antivirals | Tenofovir, entecavir | Hepatitis B |
| Antibiotics | Amoxicillin, azithromycin | Bacteria only — no effect on viruses |
Valaciclovir is aciclovir in a form the body converts more efficiently, so it can be taken less often — useful for adherence, though aciclovir remains widely used and effective. See the HIV and antiviral medicines hub.
What to check before using Aciclovir
Start it as early as possible. For a recurrent cold sore or genital outbreak, that means at the first tingling or burning — before blisters appear if you can.
Drink plenty of water while taking tablets, particularly at higher doses. This is the single most important practical instruction with oral aciclovir: it is cleared by the kidneys and can crystallise in them if you are dehydrated.
Take the full course, at evenly spaced times through the day.
Tell your doctor or pharmacist if you:
- Have kidney disease. This is the key check — aciclovir is cleared by the kidneys and the dose usually needs reducing, sometimes substantially.
- Are elderly, where kidney function is often reduced.
- Are dehydrated, or unable to drink normally.
- Have a weakened immune system, where different doses and longer courses are used.
- Are pregnant or breastfeeding. Aciclovir is used in pregnancy where needed — discuss it.
- Take probenecid, ciclosporin, tacrolimus or other medicines affecting the kidneys, including anti-inflammatories such as ibuprofen.
- Are giving it to a child, where dosing is by weight.
For the cream: apply to the affected area only, wash hands before and after, and do not use it in the eyes, mouth or vagina. Do not share it, and use your own towel and flannel during an outbreak.
Possible cautions and when to ask a pharmacist or doctor
Aciclovir is generally very well tolerated. Common effects with tablets are nausea, vomiting, diarrhoea, abdominal pain, headache and dizziness. The cream may cause local stinging, burning or dryness.
Seek medical attention if you develop:
- Much less urine than usual, or swelling of the legs — possible kidney effects.
- Confusion, hallucinations, agitation, tremor or unsteadiness. Neurological effects are recognised, particularly at higher doses, in older people, and where kidney function is impaired — they usually resolve when the dose is adjusted, but need reporting.
- A seizure.
- Swelling of the face, lips or tongue, or difficulty breathing.
- A severe rash, blistering or peeling skin.
- Yellowing of the eyes or skin.
- Unusual bruising or bleeding.
Seek medical attention promptly for:
- Shingles affecting the eye, forehead or nose tip — this can threaten sight and needs same-day assessment.
- Shingles with severe pain, or in someone with a weakened immune system.
- A cold sore spreading widely, or affecting the eye.
- Severe headache with fever, confusion or drowsiness — herpes encephalitis is a medical emergency.
- A first episode of genital herpes, which should be assessed and is worth testing for other infections at the same time.
- Chickenpox in an adult, in pregnancy, or in someone immunocompromised.
- An outbreak that is not improving after a few days of treatment.
On living with recurrent herpes: outbreaks typically become less frequent over time, and suppressive treatment substantially reduces both outbreaks and the risk of passing the virus to a partner. If recurrences are frequent or distressing, that is worth discussing — it is a common, manageable situation rather than something to endure quietly.
Related medicine guides
Frequently asked questions
What is aciclovir used for? Herpes virus infections — cold sores, genital herpes, shingles and chickenpox — both treating outbreaks and preventing recurrences.
What is aciclovir 400 mg used for? A common strength for genital herpes treatment and for suppressive therapy. Your doctor sets the dose and schedule.
What is aciclovir 200 mg used for? The lower tablet strength, used for treating herpes simplex outbreaks and for suppression.
Does it cure herpes? No. The virus lies dormant in nerve cells between outbreaks, where aciclovir cannot reach it. It treats and suppresses outbreaks.
When should I start taking it? As early as possible — at the first tingling or burning, ideally within 24 to 72 hours. Started late, the benefit is much smaller.
Does it treat HIV? No. Aciclovir works on herpes viruses. HIV needs antiretroviral medicines.
Why must I drink plenty of water? Aciclovir is cleared by the kidneys and can crystallise in them if you are dehydrated, particularly at higher doses.
Can it stop me passing herpes to my partner? Suppressive treatment substantially reduces the risk, though it does not eliminate it. Discuss it with your doctor.