Quick answer
What is aciclovir and antivirals used for?
Aciclovir and related antivirals (valaciclovir, famciclovir) treat herpes simplex (cold sores and genital herpes) and shingles by stopping virus replication. Cream works for early cold sores; tablets are needed for genital herpes, shingles, and frequent recurrences. Start as soon as possible in outbreaks. Generally safe — drink fluids; caution in kidney disease. Available on NHS prescription.
Antiviral medicines for herpes and shingles
Aciclovir revolutionised treatment of herpes simplex (HSV) and varicella-zoster (VZV) viruses in the 1980s. Valaciclovir and famciclovir are prodrugs — converted to active form in the body — allowing less frequent dosing and better absorption.
They do not cure latent virus — they shorten outbreaks, reduce severity, and lower transmission when used suppressively.
How antivirals work
Block viral DNA polymerase — prevent virus replicating during active infection.
Ineffective against latent virus hiding in nerve ganglia — explains recurrences when stopped.
Aciclovir cream — local treatment
5% aciclovir cream (Zovirax):
- cold sores — apply at prodrome
- 5 times daily for 5 days
- modest benefit if early
Not for genital herpes — use oral tablets.
Oral aciclovir — tablets
Indications:
- genital herpes first episode and recurrences
- severe cold sores
- chickenpox in high-risk patients
- eczema herpeticum — emergency
Typical doses:
- 400mg five times daily — 5 days outbreak
- 200 to 400mg two to four times daily — suppression
Frequent dosing — adherence challenge — valaciclovir often preferred.
Valaciclovir (Valtrex)
Converted to aciclovir after absorption:
| Use | Dose |
|---|---|
| Genital herpes outbreak | 500mg bd × 5 days |
| First episode genital | 1g bd × 10 days |
| Suppression | 500mg od |
| Shingles | 1g tid × 7 days |
| Cold sore (oral) | 2g bd × 1 day (single-day regimen) |
Better bioavailability — fewer doses — NHS first-line for many indications.
Famciclovir (Famvir)
Alternative prodrug — similar spectrum:
- shingles — 500mg three times daily 7 days
- genital herpes — outbreak and suppression regimens
Choice often cost and GP preference.
Shingles — time-critical
Start within 72 hours of rash:
- valaciclovir 1g three times daily 7 days
- reduces post-herpetic neuralgia duration
- see shingles guide
Immunocompromised — may need IV aciclovir in hospital.
Suppressive therapy — genital herpes
Daily valaciclovir 500mg if ≥6 recurrences/year:
- reduces outbreaks 70 to 80%
- reduces asymptomatic shedding — partner protection
See genital herpes guide.
Special situations
Pregnancy
Primary genital herpes or outbreak near labour — aciclovir/valaciclovir — specialist protocol — prevents neonatal herpes.
Kidney disease
Reduce dose — accumulation risks neurotoxicity — GP adjusts by creatinine clearance.
Hydration
Drink plenty of water — crystalluria rare historical concern.
Side effects
Common: headache, nausea
Rare: rash, liver enzyme rise, confusion in elderly/dehydrated
Stop and seek help: allergic reaction, reduced urine output
Interactions
Generally few — probenecid increases levels — nephrotoxic drugs — caution with renal function.
What antivirals cannot do
- eliminate latent virus
- replace condoms for STI prevention entirely
- treat bacterial STIs — chlamydia needs antibiotics
Early tablets beat late tablets — stock prescription for recurrent genital herpes or cold sores if GP agrees patient-initiated treatment at prodrome.
Common questions
- What is the difference between aciclovir and valaciclovir?
- Aciclovir (Zovirax) — taken multiple times daily; valaciclovir (Valtrex) is a prodrug converted to aciclovir with better oral absorption — twice daily dosing for outbreaks, once daily for suppression. Famciclovir (Famvir) — similar use, three times daily for outbreaks.
- How quickly should I start antivirals for a cold sore?
- Cream at first tingling — 5 times daily for 5 days. Tablets if severe or frequent — aciclovir 200 to 400mg five times daily for 5 days started within 48 hours of onset. Earlier start shortens outbreak more.
- What dose of valaciclovir for genital herpes?
- Outbreak — 500mg twice daily for 5 days (or 1g twice daily for 10 days first episode). Suppression — 500mg once daily if 6+ recurrences yearly. Adjust if kidney problems — GP calculates dose.
- What are side effects of aciclovir?
- Generally well tolerated — headache, nausea, diarrhoea. Rare — kidney problems if dehydrated or high dose in renal failure — maintain hydration. Allergic reactions rare. No significant interaction with alcohol in moderation.
- Can I take aciclovir in pregnancy?
- Aciclovir used in pregnancy for primary genital herpes and near delivery to prevent neonatal herpes — obstetric team guides. Safety data reassuring for indicated use — do not leave genital herpes untreated in pregnancy.