When aciclovir does not seem to be working, resistance is usually not the first explanation. In people with a normal immune system, aciclovir resistant herpes simplex virus is uncommon and is generally found in less than 1% of cases. The more common reasons are starting treatment too late, using a treatment that is not right for the infection, missing doses, or treating a sore that is not actually caused by herpes.
This guide explains why aciclovir may not seem to work, how it acts on the virus, and what to do if your symptoms do not improve. It covers tablets and cream, including treatment for cold sores and shingles.
Key things to know
- Aciclovir works best when treatment is started as early as possible. For recurrent herpes, this can mean starting during the tingling stage or when the first lesions appear.
- Aciclovir does not remove the herpes virus from your body. A later outbreak does not automatically mean that treatment has failed.
- Aciclovir strengths are not interchangeable. Doses such as 200mg, 400mg and 800mg are used in different treatment regimens.
- Aciclovir resistance is uncommon in people with a normal immune system, but it is more of a concern in people with a weakened immune system.
- Aciclovir is cleared by the kidneys, so drinking enough fluid is important while taking the tablets, particularly if you have kidney problems.
- If a cold sore is not healing or your symptoms are getting worse despite treatment, speak to a pharmacist or prescriber rather than simply starting another course.
How long does aciclovir take to work
Whether you searched how long does it take for acyclovir to work, how long does acyclovir take to work or how long does it take for aciclovir tablets to work, there is an important difference between the medicine starting to act and the sore looking better.
Aciclovir starts working against the virus after it is absorbed into the body, but that does not mean a cold sore will disappear within a few hours. The medicine stops herpes viruses from multiplying, which can help shorten and reduce the severity of an outbreak.
The timing of treatment matters. Aciclovir should be started as soon as possible after symptoms begin. For recurrent herpes infections, treatment is preferably started during the early tingling or burning stage, or when the first lesions appear. Starting treatment later may mean there is less benefit because the infection has already progressed.
If you expected the blister to disappear overnight, the lack of an immediate change does not necessarily mean the medicine has failed. The skin still needs time to heal.
Why is acyclovir not working
The wording varies, from acyclovir not working and aciclovir not working to aciclovir tablets not working, acyclovir is not working and why is acyclovir not working. The possible reasons are usually quite straightforward.
| Likely reason | What to consider |
|---|---|
| Started too late | Treatment works best when started as early as possible |
| Wrong strength or regimen | Different infections need different doses and treatment schedules |
| Doses missed or the course stopped early | Missing doses can reduce the amount of medicine available to control the virus |
| It is not a cold sore | Other skin conditions can look similar but will not respond to aciclovir |
| Expecting it to prevent every future outbreak | Aciclovir treats the infection but does not remove the virus from the body |
| Genuine resistance | This is uncommon in people with a normal immune system but can be more likely in people with a weakened immune system |
Started too late, which is a common reason
Aciclovir stops herpes viruses from making copies of themselves. This is why timing matters. For recurrent herpes infections, treatment is best started during the early tingling or burning stage, or when lesions first appear.
If blisters have already developed, it can still be appropriate to start treatment, depending on the infection and the treatment prescribed. However, starting later may mean the medicine has less effect than it would have had if started earlier.
Keeping treatment available can be useful for people who regularly get recurrent cold sores, as it means they can seek treatment promptly when symptoms begin. You can read more about our cold sore treatment assessment.
The strength may be wrong for the condition
This can cause confusion because aciclovir comes in several strengths. Aciclovir 200mg, 400mg and 800mg tablets are not simply different sized versions of the same treatment. The dose and schedule depend on the infection being treated.
| Strength | Typically used for |
|---|---|
| 200mg | Herpes simplex infections, including genital herpes |
| 400mg | Some herpes simplex treatment and suppression regimens |
| 800mg | Shingles and some chickenpox treatment regimens |
For example, 800mg taken five times a day is a recognised regimen for shingles. It is not the usual oral regimen for a simple recurrent cold sore.
The correct dose depends on the infection, your health and other factors. Follow the instructions from your prescriber or the medicine leaflet rather than changing the dose yourself.
It may not be a cold sore at all
Aciclovir works against certain herpes viruses. If a sore has another cause, aciclovir will not treat that underlying problem.
Several conditions can look similar to a cold sore. Angular cheilitis can cause sore, cracked corners of the mouth. Impetigo is a bacterial skin infection. Contact dermatitis can happen after using a new lip product, cosmetic or other irritant. Chapped lips and mouth ulcers can also be mistaken for cold sores.
A cold sore often starts with tingling, burning or itching before small blisters appear. If a sore looks different from your usual cold sores, keeps changing, or does not improve as expected, ask a pharmacist or prescriber to check it.
Doses missed or the course stopped early
Aciclovir tablets are usually taken several times a day because the medicine is cleared from the body relatively quickly. Missing doses can reduce the amount of medicine available to control the virus.
If you have been prescribed a course, take it as directed. Do not stop early simply because the sore looks better unless a healthcare professional tells you to do so.
If you forget a dose, check the leaflet supplied with your medicine or ask a pharmacist what to do. Do not take a double dose unless you have been specifically told to.
Acyclovir Not Working Anymore: Understanding the Reasons
If aciclovir worked during a previous outbreak but you have had another cold sore since then, this does not necessarily mean that the medicine has stopped working.
Aciclovir treats an active herpes infection. It does not remove herpes simplex virus from the body. After an infection, the virus can remain inactive in nerve cells and become active again later. Triggers can include illness, stress, tiredness and exposure to sunlight, although triggers vary from person to person.
A new cold sore after a previous successful course can therefore be a new outbreak rather than treatment failure.
If your outbreaks are becoming more frequent, last longer than usual or are more severe, speak to a healthcare professional. They can check whether the diagnosis is correct and whether a different treatment approach may be appropriate. For some people with frequent recurrent herpes infections, daily suppressive antiviral treatment may be considered.
People with a weakened immune system may be more likely to develop persistent or more severe herpes infections, and antiviral resistance is also more of a concern in this group. Your prescriber may need to consider your immune health, kidney function, other medicines and the type of infection before deciding on treatment.
How common is acyclovir resistance
Aciclovir resistance is uncommon in people with a normal immune system. Reviews of the available evidence generally put resistance below 1% in this group. Some older studies have reported figures around 0.1% to 0.6%.
Resistance is more common in people with a weakened immune system, including some people receiving intensive immune suppressing treatment or certain transplant patients. Persistent herpes lesions that do not respond to antiviral treatment may need further assessment in these situations.
This is why resistance should not usually be the first assumption when aciclovir does not seem to work. Timing, adherence, the diagnosis and the treatment regimen are more practical things to check first.
How does acyclovir work for cold sores
Aciclovir is an antiviral medicine. It works by interfering with the herpes virus as it makes copies of its genetic material.
The medicine is activated inside infected cells through a process that involves a viral enzyme called thymidine kinase. Once activated, aciclovir interferes with the viral DNA polymerase and stops the virus from extending its new DNA chain normally.
This helps explain why timing matters. Aciclovir has its effect on virus that is actively multiplying, so starting treatment early can give it a better chance to control the outbreak.
Resistance can develop when changes in the virus affect the enzymes or processes needed for aciclovir to work. Changes involving the viral thymidine kinase are an important cause of aciclovir resistance.
Aciclovir tablets 800mg five times a day, and herpes zoster treatment
Shingles, also called herpes zoster, is caused by the varicella zoster virus. This is the same virus that causes chickenpox. Aciclovir can be used to treat shingles, but the dose is higher than the usual dose used for many herpes simplex infections.
For adults, the UK product information lists aciclovir 800mg five times a day, at roughly four hourly intervals while omitting the night time dose, for seven days as a treatment regimen for herpes zoster. Treatment should be started as soon as possible after the rash appears.
If you think you have shingles, seek medical advice promptly rather than waiting to see whether the rash settles on its own. Your age, health and the timing of the rash can affect the treatment decision.
Side effects of acyclovir 800mg
Aciclovir can cause side effects, although not everyone gets them. The possible effects can include headache, dizziness, nausea, vomiting, diarrhoea, stomach discomfort, tiredness, itching and a skin rash.
Higher doses and reduced kidney function can increase the risk of some side effects. Drinking enough fluid is particularly important when taking higher doses, and extra care may be needed if you have kidney problems.
Seek urgent medical advice if you develop signs of a serious allergic reaction, pass very little or no urine, or develop severe confusion or unusual drowsiness. These symptoms can be more concerning in people with reduced kidney function or when higher doses are being used.
Taking the tablets with or without food is generally possible. If the medicine upsets your stomach, taking it after food may feel more comfortable.
Our guide to what to avoid while taking aciclovir covers possible interactions and practical precautions in more detail.
What to do if acyclovir does not work
If you have finished a course and your symptoms have not improved as expected, do not simply increase the dose or start another course yourself.
Instead:
- Check when you started treatment. Starting later can reduce the benefit, particularly with recurrent herpes infections.
- Check whether you missed any doses or stopped the course early.
- Think about whether the sore looks like your usual cold sore. If it looks different, another condition may be responsible.
- Check that you are using the treatment prescribed for your particular infection. Cold sores, genital herpes and shingles can have different treatment regimens.
- Speak to a pharmacist or prescriber if your symptoms are not improving, are getting worse or keep coming back.
- Tell the healthcare professional about any condition or medicine that affects your immune system or kidney function.
Do not double a dose to make up for a missed dose, take a second course without advice, or add another antiviral medicine yourself.
If cold sores are happening often enough to affect your everyday life, a prescriber can discuss whether another treatment approach, such as suppressive antiviral treatment, may be suitable.
When to get medical advice
Speak to a pharmacist, GP or other healthcare professional if:
- Your cold sore is not healing as expected despite treatment.
- You have finished your prescribed course and your symptoms are still present or getting worse.
- This is your first suspected outbreak, or the sore looks different from your usual cold sores.
- A cold sore is near your eye, or you have eye pain or changes to your vision. These symptoms need prompt medical assessment.
- You are getting frequent outbreaks, particularly if they are affecting your daily life.
- You have a weakened immune system because of a health condition or medicines such as chemotherapy or some immune suppressing treatments.
- You develop severe or persistent side effects.
If you are caring for a newborn, take extra care if you have an active cold sore. Herpes simplex can cause serious illness in newborn babies. Avoid kissing a baby while you have an active cold sore and wash your hands after touching the affected area.
Buying aciclovir tablets in the UK
Aciclovir cream for cold sores is available from pharmacies without a prescription in some pack sizes. Aciclovir tablets are prescription only medicines in the UK, so they require an appropriate assessment and prescription before they can be supplied.
| Treatment | From |
|---|---|
| Aciclovir 5% Cream | £8.99 |
| Aciclovir Tablets | £12.99 |
| Valaciclovir | £18.99 |
| Valtrex | £34.99 |
Prices were correct at the time of writing. The live product pages should always be used for current prices and availability.
Valaciclovir is converted into aciclovir in the body and is absorbed more efficiently. This allows it to be taken less frequently in some treatment regimens. Valtrex is a branded form of valaciclovir, so the active ingredient is valaciclovir rather than aciclovir itself.
If you want to buy aciclovir tablets or discuss whether tablets or cream may be appropriate for your symptoms, our cold sore treatment consultation is reviewed by a UK prescriber before treatment is supplied.
You can also read our guide to aciclovir side effects and what to avoid. If you are pregnant or planning a pregnancy, see our aciclovir in pregnancy guide for more information.
Medical Disclaimer
This page provides general information about aciclovir, a medicine available in the UK. It is not medical advice, a diagnosis or a substitute for an assessment by a pharmacist, GP or other appropriately qualified prescriber who knows your medical history. EverydayMeds is not the manufacturer of aciclovir, and this is not the official product website.
No individual dose is recommended on this page. Where strengths and treatment regimens are mentioned, they are included to explain how aciclovir may be used for different infections and are not instructions for you to follow. Always take aciclovir exactly as prescribed and follow the patient information leaflet supplied with your medicine. Do not double a dose to make up for a missed dose or start another course without speaking to a healthcare professional.
Resistance figures come from published studies of particular populations and do not predict what will happen in an individual person. Treatment responses can vary. If aciclovir does not seem to be helping, this does not by itself show that the virus is resistant or that the medicine has failed. A pharmacist, GP or prescriber can help work out the most likely reason and decide whether further assessment or treatment is needed.
