Aciclovir

What are Aciclovir 400mg tablets?

Aciclovir 400mg tablets are a prescription-only antiviral medicine. The active substance, aciclovir, belongs to a class of medicines called nucleoside analogues. For cold sores (herpes labialis), caused by the herpes simplex virus, aciclovir works by getting inside virus-infected cells and blocking the enzyme the virus needs to copy its own genetic material. With replication interrupted, the outbreak is brought under control more quickly than it otherwise would be.

Aciclovir tablets do not remove the herpes simplex virus from the body. Once acquired, the virus stays dormant in nerve tissue between outbreaks, which is why cold sores can return. What the tablets can do is shorten and calm down an active outbreak, or, taken regularly, reduce how often outbreaks happen.

Why would someone need tablets rather than cream?

For most people, an occasional, mild cold sore is usually managed first with a topical cream applied directly to the lip. Tablets are a different type of treatment: aciclovir is absorbed into the bloodstream and works throughout the body, rather than only at the site of the sore.

A UK-registered prescriber may consider tablets more appropriate than cream in situations such as:

• Frequent outbreaks, for example six or more a year
• Outbreaks that are unusually severe, widespread, or slow to heal
• A weakened immune system, where outbreaks can be harder for the body to bring under control on its own

This isn't a checklist for self-selecting treatment. Whether tablets, cream, or both are the right approach depends on an individual assessment, and it's the prescriber's judgement to make during your online consultation, based on your history and how your cold sores tend to behave.

How are Aciclovir 400mg tablets dosed?

Dosing for cold sores generally follows one of two patterns. For a single active outbreak, a short course is typically used to bring the episode under control. For people who experience frequent recurrences, a lower-dose, longer-running course may be considered to reduce how often outbreaks occur.

The exact dose, frequency, and length of course are set by your UK-registered prescriber based on your individual circumstances, including how often you get cold sores, their severity, and any other health conditions. This page is not a self-dosing guide, and you should always follow the instructions given by your prescriber and included on your dispensing label rather than adjusting the dose yourself.

When to start taking it

Aciclovir tablets work best when started as early as possible in an outbreak. Many people experience a warning sign before a cold sore becomes visible, such as tingling, itching, or a slight burning sensation around the lip. Starting treatment at this stage, rather than waiting for a blister to form, gives the medicine the best chance of shortening the episode. If you recognise these early signs and have a course of tablets available, starting promptly is generally more effective than starting once the sore is already established.

Who shouldn't take it, or should take extra care

Aciclovir tablets aren't suitable for everyone. You should tell your UK-registered prescriber if you have, or have ever had:

• Reduced kidney function, as aciclovir is cleared from the body via the kidneys and doses may need to be adjusted
• A known allergy to aciclovir, valaciclovir, or any of the ingredients listed in the patient information leaflet

Aciclovir should only be used in pregnancy or while breastfeeding if your prescriber judges that the benefits outweigh the risks, so it's important to mention if either applies to you. Full details of who should exercise caution are set out in the patient information leaflet, and your prescriber will review your medical history before confirming a prescription.

Common side effects

As with all medicines, aciclovir tablets can cause side effects, though not everyone experiences them. The most commonly reported include:

• Nausea
• Vomiting
• Diarrhoea
• Abdominal discomfort
• Headache
• Dizziness
• Tiredness

These are usually mild and don't necessarily mean the medicine isn't working.

Seek urgent medical attention if you experience signs of a severe allergic reaction, such as swelling of the face, lips, tongue, or throat, or difficulty breathing. If you notice any side effects, let your UK-registered prescriber know. Suspected side effects can also be reported to the MHRA via the Yellow Card scheme. For the complete list, refer to the patient information leaflet supplied with your medication.

Interactions to be aware of

Because aciclovir is processed by the kidneys, it should be used with care alongside other medicines that can affect kidney function. Tell your prescriber about all medicines and supplements you're currently taking, including anything bought over the counter, so they can check for any relevant interactions before confirming your prescription.

Tablets versus cream: what's the difference?

Aciclovir cream is applied directly to a single sore and works locally, at the surface of the skin. Aciclovir tablets are swallowed and work systemically, circulating through the bloodstream to act on the virus more broadly. This is why tablets tend to be considered for people with more frequent or more severe outbreaks, while cream is generally used for a single, mild, occasional sore. Neither form is presented here as more effective than the other; the right option depends on your individual pattern of outbreaks, as assessed by your prescriber. You can read more about the topical option on our Aciclovir 5% Cream page, or browse our full range on the Cold Sores category page.

How to get Aciclovir 400mg tablets through DoktorABC

• Complete an online consultation – answer a short, confidential questionnaire about your medical history and your pattern of cold sore outbreaks.
• Prescriber review – a UK-registered prescriber assesses your answers and, if tablets are considered appropriate, approves a prescription.
• Delivery – your medication is dispensed by a GPhC-registered pharmacy and delivered to your door in discreet packaging.

DoktorABC UK is a digital health platform that connects patients with independent, UK-registered prescribers. DoktorABC UK does not itself provide medical care, issue prescriptions, or dispense medicines. All clinical decisions are made by the UK-registered prescriber following an individual medical assessment.

Aciclovir 400mg tablets patient information leaflet via the Electronic Medicines Compendium (eMC).

The PIL in your medicine pack may differ from the version on this website, as it may have been updated since your medicine was packaged or the medicine may be from another brand. This page provides general information only and does not replace the PIL or individual medical advice.