Anti-infective
Antiviral drugs (herpes)
Aciclovir and related antivirals — Treat cold sores, genital herpes and shingles — most effective when started early.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
These antivirals treat infections caused by the herpes family of viruses — cold sores, genital herpes, chickenpox and shingles — and are used both for active episodes and, sometimes, to suppress frequent recurrences.
How it works
They are activated inside virus-infected cells and then block the viral enzyme that copies its DNA, stopping the virus from multiplying. Because they target an infected-cell process, they spare healthy cells and are generally well tolerated.
In practice
In practice the single biggest determinant of benefit is timing: these antivirals work best when started within the first day or two of symptoms (or at the very first warning tingle of a recurrence), so prompt access matters. They are generally very well tolerated; the main practical points are keeping the patient well hydrated and reducing the dose in renal impairment, since the kidneys clear the drug. In shingles, early treatment of those who qualify can also reduce the risk of lingering nerve pain.
Examples
Practical use
How to take it & use it well
- Start aciclovir or valaciclovir as early as possible once symptoms or tingling begin, because these antivirals work best when started in the first hours to days of an outbreak.
- Take every dose evenly spaced through the day and complete the full course your clinician prescribes, even if the rash or sores start to improve.
- Drink plenty of fluids while taking these medicines to keep well hydrated and protect your kidneys.
- You can usually take them with or without food; valaciclovir is taken less often than aciclovir because the body converts it efficiently.
- If a dose is missed, take it when you remember unless it is nearly time for the next one, then continue as normal without doubling up.
- For cold sores, antiviral creams work best applied at the very first tingle, while tablets are used for more severe or frequent outbreaks under clinical advice.
Common uses
- Cold sores and genital herpes (treatment and suppression)
- Shingles (herpes zoster)
- Chickenpox in those at higher risk
Monitoring
- Renal function with higher-dose or IV treatment
- Symptom response and hydration
- Frequency of recurrences if used for suppression
Weighing it up
Advantages & disadvantages
Advantages
- Started early, they shorten outbreaks of cold sores, genital herpes and shingles and ease pain.
- They have a long track record and are generally well tolerated.
- Regular low-level use can reduce how often genital herpes recurs for people troubled by frequent outbreaks.
- Prompt treatment of shingles can lower the risk of lingering nerve pain afterwards.
Disadvantages
- They control outbreaks but do not cure or remove the virus from the body.
- They work much less well if started late in an outbreak.
- Doses often need adjusting if your kidneys are not working well, and dehydration raises the risk of kidney problems.
- Some people get headache, nausea or feeling generally off-colour while taking them.
Key safety principles
What to watch for
- Reduce the dose in renal impairment and maintain good hydration.
- Most effective when started early — ideally within 24–72 hours of symptoms.
- Generally well tolerated; high or intravenous doses need particular attention to kidney function.
Key interactions
What to avoid or check alongside
- Other medicines that can stress the kidneys, such as certain anti-inflammatory painkillers, may add to the risk of kidney problems.
- Probenecid can slow removal of these antivirals from the body, raising their levels.
- Mycophenolate, used after transplants, may interact and needs monitoring when used together.
- Theophylline levels can rise with aciclovir, increasing the chance of side effects like a fast heartbeat or jitteriness.
- Being dehydrated, including from diuretics or illness, increases the risk of antiviral-related kidney effects.
Patient & carer advice
- Start treatment as early as possible — at the first tingle for recurrent cold sores or genital herpes
- Drink plenty of fluids while taking it
- These reduce severity and duration but do not cure the underlying virus
Use with
Related clinical calculators
Dose and risk decisions for this class often depend on renal function, weight or bleeding/stroke risk. These tools help:
Answers
Antiviral drugs (herpes): frequently asked questions
Do antivirals cure herpes or shingles?
No. Aciclovir and valaciclovir control outbreaks and speed healing but do not clear the virus from the body, which stays dormant. They shorten symptoms and can reduce how often herpes recurs, but the virus can reactivate later.
Why is starting early so important?
These antivirals stop the virus from multiplying. The virus is most active at the start of an outbreak, so treatment started within the first hours to days works far better than treatment begun once sores are well established.
Do I need to drink extra water with these medicines?
Yes, staying well hydrated is important because these antivirals are cleared by the kidneys and can occasionally affect them, especially if you are dehydrated. Drink plenty of fluids throughout the course unless your clinician advises otherwise.
Can I take herpes antivirals in pregnancy?
Aciclovir and valaciclovir are commonly used in pregnancy when needed and are generally considered acceptable, but you should always discuss it with your midwife, GP or pharmacist so your treatment is tailored to your situation.
Why might my dose be different from someone else's?
Doses depend on what is being treated, how severe it is, and how well your kidneys work. People with reduced kidney function often need adjusted dosing, which is why these medicines should be taken as prescribed for you.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
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