There is a reason people search "aciclovir 400mg how to take" rather than simply reading the box. Aciclovir 400mg tablets are usually taken by swallowing them whole with water. They can also be dissolved in a glass of water, stirred and then drunk. You can take them with or without food. The exact timing and number of doses should follow the label supplied by your pharmacy, with doses spaced as directed for the condition being treated.
Aciclovir 400mg is not used for just one condition. The same strength can be prescribed at different frequencies and for different lengths of time, including for the treatment or prevention of genital herpes, other herpes virus infections, or prevention of infection in some people with weakened immune systems. This means the leaflet inside the box lists several possible regimens, while the label printed by your pharmacy gives the instructions that apply to you. When the two do not seem to match, it is understandable to look for another explanation.
This page explains how to take the tablets, answers the question about taking aciclovir with food, explains why the leaflet and pharmacy label can look different, and clears up a common misunderstanding about what 400mg is used for. It also covers which conditions this strength may be used for, how dosing schedules are usually organised, why staying hydrated matters, and what to do if you miss a dose.
What it does not do is give you one universal dose. That is because aciclovir 400mg is used in more than one way. The right dose, timing and treatment length depend on the condition being treated and your individual circumstances. Your pharmacy label is therefore the instruction that applies to you.
Key Things to Know
- Aciclovir tablets can be taken with or without food
- Swallow the tablet whole with water, or dissolve it in a glass of water, stir and drink it
- Drink plenty of water throughout the course, especially if you have been advised to take extra care with your fluid intake
- Doses are usually spaced through the waking day, and the number of doses depends on the condition being treated
- 400mg is not usually the strength prescribed for oral cold sore treatment in UK practice
Start Here: Your Label, Not the Leaflet
The most useful thing to remember is this: the instruction that applies to you is the one printed on your pharmacy label.
The patient information leaflet inside a box of aciclovir 400mg tablets is written to cover all the licensed uses of that strength. It can therefore list several different regimens for treating one condition, preventing another, or for people whose immune systems are not working normally. The leaflet explains that the dose depends on why you have been prescribed the medicine.
Your pharmacy label, by contrast, has been prepared specifically for you based on your prescription.
When They Appear to Contradict Each Other
This can be confusing. If the leaflet describes a regimen that does not match your pharmacy label, follow the label supplied with your medicine. The leaflet is not necessarily wrong. It is giving general information that covers several different uses.
If something genuinely does not make sense, or if the label is missing, damaged or unclear, do not guess or change the dose yourself. Contact your pharmacy, doctor or pharmacist for advice.
What Is Aciclovir 400mg Actually For?
Aciclovir is an antiviral medicine used to treat certain viral infections caused by the herpes simplex virus and varicella zoster virus. These include cold sores, genital herpes, chickenpox and shingles. It comes in several tablet strengths, and the strengths are not simply smaller and larger versions of the same treatment. Different strengths can be used for different conditions and treatment plans.
| Strength | What it is typically used for |
|---|---|
| 200mg | Herpes simplex infections, including oral treatment of cold sores, and some genital herpes regimens |
| 400mg | Genital herpes treatment and longer term suppression, and prevention in some people whose immune systems are not working normally. For example, prescribed aciclovir 400mg may be used to treat genital herpes, although the exact regimen depends on the individual and the condition being treated |
| 800mg | Shingles and chickenpox, and certain short course genital herpes regimens |
The Correction That Matters Most
You may find websites, including some pharmacy websites, saying that aciclovir 400mg is the standard treatment for cold sores. That is not usually the position in UK practice. Where oral treatment is prescribed for cold sores, 200mg is commonly used in UK practice, although the appropriate strength depends on the clinical situation. Treatment may be more useful when started early, as advised by a healthcare professional.
This does not mean that a prescriber is wrong to give you 400mg. There may be a clinical reason for choosing that strength. It does mean that if you are reading a general article claiming that 400mg is the routine cold sore dose, you should not use that article to change your treatment. Follow your own pharmacy label instead.
Do not change the strength yourself, whether by halving a higher strength tablet or taking two lower strength tablets. Ask your pharmacist or prescriber if you are unsure.
Can You Take Aciclovir on an Empty Stomach?
Yes. Aciclovir tablets can be taken with or without food.
Food does not have a significant effect on how aciclovir is absorbed, so you do not need to plan your doses around meals. You also do not need to eat first if you would rather not. This can be helpful when your prescription involves several doses during the day and the timings do not fit neatly around meals.
If aciclovir makes you feel slightly sick, you may find that taking it with some food is easier on your stomach. This is about comfort rather than making the medicine work better.
How to Take the Tablets
Swallowing or Dissolving
You have two options described in NHS guidance. You can swallow the tablet whole with a drink of water, or dissolve it in a glass of water, stir it and drink it.
The second option can be useful if you find tablets difficult to swallow. If you dissolve the tablet, drink the full glass. You can also add a little more water afterwards and drink that to help make sure you have taken the full amount.
Water, and Why It Matters More Than You Think
NHS guidance recommends drinking plenty of water while taking aciclovir tablets and taking particular care if you have kidney problems.
Aciclovir is removed from the body mainly through the kidneys. Staying well hydrated may help reduce the risk of kidney related problems, particularly in people who are more vulnerable to them.
There is no official number of glasses that everyone needs to drink. A practical guide is to drink regularly throughout the day and pay attention to signs that you may be dehydrated. Your urine is usually pale yellow when you are well hydrated. You may need to take extra care with fluids during hot weather or if you are unwell with vomiting or diarrhoea.
If you have been told to restrict your fluid intake because of a medical condition, follow that advice rather than increasing your fluids without speaking to your healthcare professional.
Spacing Doses Through the Day
Depending on what it is treating, aciclovir can be prescribed between two and five times a day. Your doses should be spaced as evenly as possible according to the instructions on your label.
If you need several doses a day, planning the times in advance or setting an alarm can make it easier to remember. Do not move doses closer together or take extra tablets just to make the schedule more convenient.
Finishing the Course
Take aciclovir for the full length of time prescribed, unless your prescriber tells you to stop. Even if your symptoms improve after a few days, do not change the treatment schedule yourself.
It is also best not to use leftover aciclovir from a previous course for a new problem without getting advice. The strength, dose and treatment length that were appropriate for one episode may not be right for another.
How Many Days Do You Take It For?
The honest answer is that it depends on what you are being treated for, and the range can be quite wide.
NHS guidance describes treatment courses that commonly last around five to ten days. Treatment used to help prevent recurrences may continue for a longer period, sometimes many months, with regular review.
As one example of suppressive treatment, some adults may be prescribed aciclovir 400mg twice daily for an extended period, with regular review to help reduce future episodes. However, the right aciclovir dosage depends on the condition being treated and the prescriber's advice.
That is why a search such as "aciclovir 400mg dosage for adults, how many days" does not have one answer. A short treatment course and a longer suppressive course can use the same tablet but have very different instructions. Your label should state how long you need to take it. If it does not, ask the pharmacy.
For recurrent herpes infections, antiviral treatment is generally most useful when started early in an episode. If you have recurrent outbreaks, follow the plan given by your prescriber about when to start treatment.
Missed Doses and Extra Doses
If you forget a dose, take it when you remember unless it is nearly time for your next dose. If it is nearly time, skip the missed dose and carry on with your usual schedule.
Do not take two doses at the same time or take an extra dose to make up for one you missed. If you regularly forget doses, setting an alarm on your phone may help.
If you accidentally take an extra tablet, NHS guidance says this is unlikely to cause harm in many cases. However, taking too much aciclovir over several days can cause problems. Contact your doctor or pharmacist for advice if you have taken more than prescribed. If you have taken a large amount, seek urgent medical advice through NHS 111 or another appropriate medical service.
Aciclovir 800mg: How It Differs
Since the 800mg strength often appears in the same searches, it is worth separating it clearly.
800mg is commonly used for shingles and chickenpox rather than routine cold sore treatment. It can also be used for certain other herpes simplex infections. These treatment plans can involve more frequent dosing and a higher total daily amount, which is one reason staying well hydrated is important.
Shingles treatment is generally started as early as possible after the rash appears, particularly in people who may benefit from antiviral treatment. If you think you have shingles, seek medical advice rather than delaying treatment while researching doses online.
Aciclovir helps control herpes virus infections but does not remove the virus from the body. The appropriate precautions therefore still matter where relevant.
Do not use an 800mg tablet for a cold sore simply because you have one at home. The right strength and treatment plan depend on the condition being treated.
Before You Start: What Your Prescriber Needs to Know
Aciclovir has a relatively short list of important medicine interactions, but some medicines can interact with aciclovir tablets because the medicine is removed from the body mainly through the kidneys.
The medicines that may matter include:
| Medicine | Why it matters |
|---|---|
| Probenecid | Used for gout, and it can slow the removal of aciclovir from the body |
| Cimetidine | An indigestion medicine that can affect how aciclovir is handled by the body |
| Mycophenolate mofetil | Used after transplants and for some immune conditions |
Paracetamol can generally be taken with aciclovir, but always check with a pharmacist or prescriber if you are unsure about your medicines.
Also tell your prescriber if you have kidney problems, are pregnant, planning a pregnancy or breastfeeding, are older, have previously reacted to aciclovir or valaciclovir, or take any other medicines, vitamins or herbal remedies.
Your prescriber may need to take your kidney function and other medicines into account when deciding whether the dose needs to be adjusted.
Side Effects Worth Knowing About
Many people take aciclovir tablets without serious problems, but side effects can occur. These may include headache, dizziness, feeling sick, diarrhoea, rash and tiredness.
More serious reactions are less common. Seek medical advice promptly if you develop signs of a serious allergic reaction, yellowing of the skin or eyes, a marked reduction in urine output, unusual bruising or recurrent infections.
It is also worth remembering that some of the more alarming information about aciclovir online relates to intravenous treatment given in hospital. That is different from taking a prescribed course of oral tablets at home.
With aciclovir cream, mild burning or stinging can occur where the cream is applied.
If It Does Not Seem to Be Working
Aciclovir does not clear herpes infections from the body. It acts on herpes viruses while they are replicating, so it may help shorten or reduce the severity of an episode, but it does not remove the virus from the body.
If an episode is not improving as expected, there can be several possible reasons. Treatment may have been started later than recommended, the episode may be more severe than usual, or the symptoms may have another cause.
Resistance to aciclovir is uncommon in people with a healthy immune system, but it is more likely in people with a weakened immune system.
If your symptoms are not improving or are getting worse, speak to your prescriber rather than increasing the dose yourself.
Treatment Options
Treatment suitability depends on an individual clinical assessment. If aciclovir tablets may be appropriate for you, treatment usually starts with an online consultation before a prescriber decides whether treatment is suitable and, if so, the appropriate strength and duration.
| Treatment | From |
|---|---|
| Aciclovir Tablets | £12.99 |
| Aciclovir 5% Cream | £8.99 |
Aciclovir tablets are prescription only and cannot be supplied without a prescriber review.
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 printed label specific to you and a patient information leaflet supplied 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, and it does not tell you what dose to take.
Aciclovir tablets are a prescription medicine and should only be used under the supervision of an appropriately qualified prescriber. Always follow the instructions on your pharmacy label and in the patient information leaflet supplied with your medicine. Do not start, stop, change the strength or alter the frequency of a prescribed medicine unless your prescriber advises you to do so. Do not take medicine prescribed for someone else or use leftover medicine from a previous course.
If you are unwell and not improving, your symptoms worsen, or you develop signs of a serious allergic reaction, kidney problems or jaundice, seek medical assessment promptly.
