There is a reason this question comes up so often. If you are looking for the right aciclovir 400mg dosage for cold sores, the first thing to know is that 400mg is not the usual tablet strength used for cold sores in the UK. Aciclovir 200mg tablets are more commonly prescribed, and the dose you should take is the one written on your pharmacy label.
For UK adults who have been prescribed aciclovir for cold sores, starting treatment early can be important. Taking more than prescribed is not a way to make the treatment work better and can increase the risk of side effects.
This page explains why timing matters, how the different tablet strengths are used, when treatment is usually started, how long it may last, how to use aciclovir cream, what to do if you take too much, and some reasons why treatment may not seem to work.
Key Things to Know
- Antivirals work by acting on the virus while it is replicating, which is why starting treatment early can be important
- Aciclovir is not absorbed into the bloodstream in direct proportion to the amount taken, so taking more does not necessarily make it work better
- The 200mg, 400mg and 800mg tablet strengths are used for different conditions and treatment plans, rather than simply representing different levels of severity
- Instructions for using aciclovir cream are given on the pack and explained below
- For tablets, always follow the instructions on the pharmacy label provided for you
The Short Answer, and Why It Is Not a Number
This page does not publish an oral dosing schedule for cold sores. This is deliberate.
Aciclovir tablets are a prescription only medicine in the UK. The appropriate dose can depend on what is being treated, the strength of the tablets, your kidney function and your individual circumstances. A general web page cannot assess these factors.
The same tablet strengths can also be used for different conditions and at different frequencies. This is why copying a dose from another person or from a general website can be unsafe.
If you already have aciclovir tablets, the most useful place to check is your pharmacy label. It contains the instructions written specifically for you. If the label is missing, unclear or seems different from the information in the leaflet, contact the pharmacy that supplied your medicine. They can check the prescription and explain exactly how you should take it.
This page can still help you understand how aciclovir works, why timing matters and what to do if you have questions about your treatment.
Why Timing Beats Dose for a Cold Sore
Aciclovir is an antiviral medicine. It works by interfering with the replication of herpes viruses. It works on the virus while it is actively replicating.
This is one reason timing can matter when treating a cold sore.
Cold sores often begin with a tingling, itching or burning feeling before the blister appears. Starting treatment at this early stage may help reduce the duration of symptoms.
Once the blister has appeared, the infection has already progressed. This does not mean antiviral treatment cannot be used, but starting later may be less helpful than starting early.
For that reason, it is better to follow the instructions given by your prescriber rather than increasing the dose because the cold sore looks worse. If your treatment does not seem to be working, speak to your prescriber or pharmacist rather than taking extra tablets.
Why Taking More Does Not Work Better
This is an important question because it can be tempting to think that a higher dose will make a cold sore clear faster. That is not a safe assumption.
Aciclovir Has an Absorption Ceiling
Aciclovir is not absorbed particularly well from the gut. Oral aciclovir has relatively low bioavailability, with approximately 10 to 20 per cent of a dose reaching the bloodstream.
Importantly, the amount absorbed does not increase in direct proportion to the dose. In simple terms, taking twice as much does not mean that twice as much aciclovir will reach your bloodstream.
This is one reason why you should not increase your dose yourself. The prescribed dose has been selected for the condition being treated and your individual circumstances.
Valaciclovir is a different medicine that is converted into aciclovir in the body after it has been absorbed. It was developed partly to improve the amount of aciclovir that becomes available after an oral dose. It is not a reason to take more aciclovir tablets.
What Extra Tablets Actually Do
Taking extra tablets does not necessarily make treatment more effective and can increase the risk of side effects.
Aciclovir is cleared from the body mainly through the kidneys. People with reduced kidney function may need a different dose or treatment schedule. Drinking enough fluid is also important when taking aciclovir, particularly when you are taking higher doses or have been advised to do so by your healthcare professional.
NHS guidance states that taking one extra dose is unlikely to cause harm in most situations, but taking too much over several days can cause problems. Taking significantly more than prescribed may increase the risk of side effects, including problems affecting the kidneys and, less commonly, neurological effects.
Do not increase your own dose because an outbreak looks worse. If you think your treatment is not working, speak to your prescriber or pharmacist instead.
If you have taken much more aciclovir than prescribed, seek medical advice promptly. You can contact NHS 111 for advice about what to do next.
Aciclovir Cream: The Application Guidance in Full
Aciclovir cream is different from the tablets. In the UK, some aciclovir cream products are available from pharmacies without a prescription. The exact instructions can vary by product, so always check the pack and patient information leaflet.
How to Apply It
- Apply a thin layer to the affected area and the skin immediately around it
- Use it around five times a day, roughly every four hours during waking hours, if this is what your product instructions recommend
- Wash your hands before and after applying the cream
- Use a clean fingertip or a disposable applicator if recommended by the product instructions
- Do not rub it in vigorously, and do not apply it inside the nose or mouth or near the eyes
- Start treatment as soon as possible after the first signs of a cold sore, following the instructions for your product
How Long to Use It
Use the cream for the length of time stated in the patient information leaflet or recommended by your pharmacist. If the cold sore is not improving as expected, ask a pharmacist or GP for advice.
Many cold sores improve within several days and heal within around one to two weeks.
If the sore has not healed after around ten days, speak to a pharmacist or GP rather than continuing to treat it without checking what is causing it.
One Thing Cream Cannot Do
Aciclovir cream is intended for cold sores on or around the lips and face. It should not be used inside the nose or mouth or near the eyes unless a healthcare professional has specifically advised you to use a suitable product there.
It will also not treat a skin or lip problem that is caused by something other than herpes simplex. If the sore does not look like a typical cold sore or is not improving, it is worth getting it checked.
Which Strength Is Which
The tablet strengths are not simply small, medium and large versions of the same treatment. Aciclovir is an antiviral medicine used for a number of herpes virus infections, and different strengths and treatment schedules may be used depending on the condition being treated.
| Strength | What it is generally associated with |
|---|---|
| 200mg | Commonly used for herpes simplex infections, including cold sores and genital herpes, depending on the condition and prescribing regimen |
| 400mg | Commonly used for certain genital herpes treatment and prevention regimens, depending on clinical need. It may also be used for longer term suppressive treatment in some people |
| 800mg | Commonly associated with treatment regimens for shingles and chickenpox |
The important point is that the strength does not tell you how severe an infection is. Your prescriber chooses the strength and treatment schedule based on the condition being treated and your individual circumstances.
400mg is not the usual tablet strength for oral cold sore treatment in the UK. Where oral aciclovir is prescribed for a cold sore, the prescriber will choose the strength and regimen they consider appropriate.
A prescriber may still prescribe 400mg in certain circumstances. You should follow the instructions on your own prescription rather than changing the dose based on a general dosing table.
800mg is not a standard cold sore tablet strength. It is commonly associated with treatment regimens for conditions such as shingles and chickenpox. Do not take an 800mg tablet for a cold sore simply because you already have it at home.
Never change the strength yourself by halving tablets or taking two lower strength tablets unless your prescriber or pharmacist has specifically told you to do so.
How Long Does Treatment Last?
For tablets, the length of treatment depends on the condition being treated and the regimen prescribed. An active infection may be treated for around five to ten days in some circumstances, while treatment used to prevent recurrences can continue for longer and may involve regular review.
This is why there is no single answer to how many days you should take aciclovir. Two people can have the same medicine but different treatment instructions.
Follow the course recommended by your prescriber or pharmacist unless they advise you otherwise.
If You Miss a Dose
If you forget a dose, take it when you remember unless it is nearly time for your next dose. If it is nearly time for the next dose, skip the missed dose and continue with your normal schedule.
Do not take two doses together or take an extra dose to make up for a missed one. If you are unsure what to do, check the patient information leaflet or ask your pharmacist.
If you regularly forget doses, setting an alarm on your phone may help you remember.
Does Anyone Need a Different Amount?
Yes. Some people may need a different dose or treatment schedule, which is something a prescriber should decide.
Kidney function is particularly important because aciclovir is cleared from the body through the kidneys. If your kidney function is reduced, your prescriber may adjust the dose or how often you take it.
Older adults may also need their kidney function to be considered when aciclovir is prescribed.
People with weakened immune systems may need a different treatment approach and should follow advice from their healthcare professional. This does not mean that a higher dose is automatically better.
Pregnancy, breastfeeding, other medicines you take, and any previous reaction to aciclovir or valaciclovir are also important to discuss with your healthcare professional.
If It Does Not Seem to Be Working
If your cold sore is not improving, taking more tablets is not necessarily the answer. There are several possible reasons why treatment may not seem to be working.
- Treatment was started after the early tingling stage
- The outbreak is more severe or lasts longer than your usual cold sores
- The sore is not actually a cold sore. Several conditions affecting the lips and mouth can look similar
- Doses were missed or not taken as prescribed
- The virus is less responsive to aciclovir, which is uncommon in people with a healthy immune system but can be more common in people who are significantly immunosuppressed
Aciclovir does not remove herpes simplex virus from the body. The virus can remain inactive in nerve cells between outbreaks, which is why cold sores can come back even after treatment.
If your symptoms are not improving, speak to your prescriber or pharmacist rather than changing your dose yourself.
Getting Treatment
If treatment is appropriate, a prescriber may be able to assess your suitability through an online consultation. Treatment suitability depends on an individual clinical assessment, and your prescriber will decide whether treatment is appropriate and, if so, which strength, frequency and duration are suitable for you.
| Treatment | From |
|---|---|
| Aciclovir 5% Cream | £8.99 |
| Aciclovir Tablets | £12.99 |
Prices are confirmed at checkout, and the live product page takes precedence over any figure quoted here. Medicines are dispensed by a UK pharmacy operating under GPhC regulation, with a pharmacy label specific to you and a patient information leaflet included with the medicine.
Medical Disclaimer
This article is provided for general information only and does not constitute medical advice. It is not a substitute for a consultation with a qualified healthcare professional. It deliberately does not tell you what dose of aciclovir tablets to take.
Aciclovir tablets are a prescription only medicine and should be used under the supervision of an appropriately qualified prescriber. Always follow the instructions on your pharmacy label and in the patient information leaflet.
Common side effects of aciclovir can include headache, nausea and dizziness. Check the patient information leaflet for the full list of possible side effects and precautions.
Do not increase the dose, take extra tablets, change the strength, or use medicine prescribed for someone else. Do not use leftover aciclovir from an earlier course without checking with a healthcare professional first.
If you think you have taken too much aciclovir, contact NHS 111 or speak to a pharmacist for advice. Tell your prescriber about any other medicines you are taking.
If an outbreak is not improving, is severe, involves the eye, or you have a weakened immune system, seek medical advice rather than changing your treatment yourself. If you develop signs of a serious allergic reaction, seek urgent medical attention.
