Aciclovir 400mg tablets are a widely prescribed antiviral medicine used in the UK to treat and manage herpes simplex and varicella zoster virus infections. Before starting any course, it helps to understand how the medicine works, who it may not be suitable for, and what side effects or interactions to watch for.
This guide brings together NHS and manufacturer product information on aciclovir 400mg tablets in plain English. It does not replace advice from your prescriber, who will confirm the right treatment and course length for your individual circumstances based on a full clinical assessment.
What Is Aciclovir 400mg Used For?
Aciclovir tablets are used to treat infections caused by herpes viruses, including herpes simplex infections such as genital herpes and varicella zoster infections such as shingles. The appropriate tablet strength and dosing schedule depend on the infection being treated and the individual circumstances. Aciclovir 400mg tablets may be used for some treatment and suppressive regimens, but the strength should not be selected without medical advice.
For cold sores, topical aciclovir cream or other antiviral treatment may be considered depending on the circumstances. Oral aciclovir may also be prescribed, but the appropriate strength and regimen depend on the clinical situation.
Your prescriber will select the strength and treatment that fits your diagnosis, so do not assume every aciclovir product is interchangeable. Below, you will find the key points that matter when deciding on treatment: uses, common and serious side effects, who should avoid it, drug interactions, kidney function and older-age considerations, food advice, longer-term safety, pregnancy and breastfeeding guidance, and cold sore treatment options.
Aciclovir does not cure herpes infections. The virus remains in the body between outbreaks. The virus lies dormant between episodes and can reactivate.
Aciclovir works best when treatment is started as early as possible. The ideal timing depends on the infection: for herpes simplex, treatment should generally begin as soon as symptoms or lesions appear, while shingles treatment should be started as early as possible and preferably within 72 hours of the rash appearing.
Common Side Effects of Aciclovir 400mg
Most people tolerate aciclovir tablets well, and the more common side effects that are reported include:
- Headache.
- Dizziness.
- Nausea, vomiting or diarrhoea.
- Abdominal discomfort.
- Skin rash.
- Itching.
- Tiredness and fatigue.
Most reported side effects are mild. If a side effect is troublesome, persistent or unexpected, speak to your pharmacist or prescriber rather than stopping treatment without advice.
Serious Side Effects and When to Seek Help
Some potentially serious adverse effects have been reported with aciclovir. Their frequency varies and is not known for all reactions. Contact NHS 111 or seek urgent medical help if you notice any of the following:
- Signs of an allergic reaction, such as facial or throat swelling, difficulty breathing, or a severe rash (this can indicate anaphylaxis and needs emergency treatment).
- Signs of a liver problem, such as yellowing of the skin or eyes (jaundice).
- Signs of a kidney problem: passing little or no urine.
- Signs of a blood disorder such as unusual bruising, persistent infections or unexplained severe tiredness.
Confusion, agitation, hallucinations, tremor and other neurological effects have been reported, particularly in people with significant kidney impairment or when doses are too high for kidney function. Seizures have also been reported. Seek medical advice promptly if you develop sudden confusion, unusual behaviour, severe tremor or other neurological symptoms while taking aciclovir.
Contraindications: Who Should Not Take Aciclovir
Aciclovir tablets should not be taken by anyone who has previously had an allergic reaction (hypersensitivity) to aciclovir, valaciclovir, or any of the ingredients in the tablet. Anyone with a known hypersensitivity to aciclovir or valaciclovir should not take the medicine without medical advice.
Beyond this direct contraindication, some groups need extra caution rather than an outright bar on use. These include people with existing kidney impairment, older adults, and anyone taking other medicines that affect the kidneys. Your prescriber will factor these into the assessment before confirming aciclovir is appropriate for you.
Drug Interactions and Precautions
Aciclovir is cleared by the kidneys largely unchanged, and it is not broken down by the liver in the way many other medicines are. This is why its interaction list is short compared with many drugs. The NHS highlights several medicines that may interact with aciclovir tablets, including:
- Probenecid, used for gout, which can raise aciclovir levels in the blood by competing for the same kidney elimination pathway.
- Cimetidine, used for stomach ulcers and reflux, for the same reason.
- Mycophenolate mofetil, used to prevent organ transplant rejection, where the two medicines can affect each other's blood levels via shared renal clearance.
Always tell your prescriber and pharmacist about every medicine, supplement, over-the-counter product, and other medication you take, so they can check for anything relevant to your individual case. If you use immunosuppressant medicines, other antivirals, or medicines that can affect the kidneys, this is particularly worth flagging. If you want further guidance on whether aciclovir tablets are suitable alongside other medicines, ask your prescriber or pharmacist.
The interaction considerations are different for topical aciclovir. NHS guidance does not identify medicines that affect aciclovir cream or eye ointment because very little of the medicine is absorbed into the bloodstream. Aciclovir tablets require a prescription in the UK, whereas some topical formulations can be obtained from pharmacies without a prescription.
Kidney Function and Aciclovir: What to Know
Because aciclovir is eliminated through the kidneys, kidney function is one of the main safety considerations before and during treatment. Prescribers typically take kidney health into account, particularly in older adults or anyone with a known kidney condition such as kidney disease, and this can influence the strength or course they recommend.
Because aciclovir is eliminated through the kidneys, kidney function is an important safety consideration. Aciclovir can crystallise in the kidney tubules, particularly when urine becomes concentrated, so adequate hydration is important. People with impaired kidney function may also need a lower dose or a different dosing interval. Drink plenty of fluids while taking aciclovir tablets unless your doctor has advised you to restrict your fluid intake.
Tell your doctor promptly if you notice a marked drop in how much urine you are passing, unexplained swelling, or unusual tiredness while taking aciclovir, as these can be early signs of a kidney problem worth checking. Keep taking the prescribed dose unless your doctor tells you otherwise.
Aciclovir in Elderly Patients
Older adults can take aciclovir tablets, but age-related decline in kidney function means this group is more likely to need closer monitoring or a treatment plan tailored by their prescriber. Rather than a fixed rule that applies to everyone over a certain age, the decision is based on an individual assessment, often including kidney function tests, alongside any other medicines being taken.
Older adults may also be more susceptible to neurological adverse effects, particularly when kidney function is reduced. Prescribers may therefore monitor kidney function and adjust the dose when necessary.
Aciclovir and Food: Anything to Avoid?
There is no specific food that is documented as needing to be avoided while taking aciclovir tablets. The medicine can be taken with or without food. If you find it upsets your stomach, taking it alongside a meal or snack is a reasonable comfort measure, though it is not a clinical requirement. There is no specific interaction between aciclovir and alcohol, but if you want to drink alcohol and have concerns about side effects or your overall health, check with your prescriber.
The one dietary factor that genuinely matters is fluid intake, for the kidney-related reasons covered above. Staying well hydrated is more relevant to aciclovir safety than any particular food to avoid.
Long-Term Use: Safety Considerations
Some people, particularly those using aciclovir as suppression treatment for recurrent genital herpes, take it continuously over a longer period rather than as a short course. For recurrent genital herpes, a prescriber may recommend longer-term suppressive treatment, often using 400mg twice daily.
This approach can reduce the frequency of recurrent outbreaks in people who experience frequent recurrences. Long-term use is generally well tolerated, but it is usually reviewed periodically by a prescriber to confirm it is still needed and still appropriate, especially where kidney function or other health circumstances may have changed.
If you are on a longer course, continue to report any new or worsening side effects, and attend any follow-up reviews your prescriber recommends. This is a routine part of safe long-term antiviral use rather than a sign of a specific concern.
Pregnancy and Breastfeeding
Aciclovir may be used in pregnancy or while breastfeeding, but this decision should always be made with your prescriber after assessing the potential risk for you and your baby alongside the specific infection being treated. Aciclovir may be prescribed during pregnancy when the benefits are considered to outweigh any potential risks, and available evidence has not shown an increased risk of birth defects.
Aciclovir may be used during pregnancy when clinically indicated, including for herpes infections, following an individual assessment by a healthcare professional.
Aciclovir passes into breast milk in small amounts, but the amount is generally considered unlikely to cause harm at usual maternal doses.
Cold Sore Treatment Options at EverydayMeds
Where a cold sore, rather than genital herpes or shingles, is the reason for seeking treatment, EverydayMeds offers several treatment options for cold sores, including topical aciclovir and oral antiviral treatments where clinically appropriate.
| Treatment | Standard Price From |
|---|---|
| Aciclovir Tablets | £12.99 |
| Aciclovir 5% Cream | £8.99 |
| Valaciclovir | £18.99 |
| Valtrex | £34.99 |
Prices shown are the standard price for each treatment. Exact pricing, available strengths and any current offers are confirmed at checkout. If oral treatment is considered appropriate, it can be prescribed following an online clinical assessment.
Medical Disclaimer
This article is for general information only and does not constitute medical advice. It is not a substitute for a consultation with a qualified healthcare professional. Treatment suitability depends on an individual clinical assessment. Your prescriber will determine the appropriate treatment, strength and course length for your circumstances. Results vary between individuals. If you experience symptoms that concern you, or think you may be having a serious reaction to any medicine, seek medical attention promptly or contact NHS 111.
