The exact time you take each dose of aciclovir is less important than two key things: starting treatment as soon as advised after symptoms appear, and taking your 400mg tablets exactly as your prescriber and medicine label instruct. If you are searching for information on how to take aciclovir 400mg, the answer depends on why it has been prescribed. It may be taken several times a day for a short course to treat an active herpes infection, or at a different frequency as ongoing treatment to help prevent recurrent outbreaks.
This guide is for adults in the UK who have been prescribed aciclovir 400mg tablets, or are considering treatment, for herpes simplex virus infections or prevention of recurrent outbreaks. It explains when and how aciclovir 400mg is commonly taken, how treatment for an active outbreak differs from suppressive treatment, what to know about food and missed doses, how long treatment may last, and how aciclovir is prescribed and obtained in the UK.
This is general information based on NHS guidance and approved product information. Your prescription label always takes priority because the dose and duration prescribed for you depend on the infection being treated and your individual circumstances.
Why Timing Matters More at the Start Than During the Day
Aciclovir is an antiviral medicine used to treat infections caused by herpes viruses. It works by blocking a viral enzyme that the virus needs to replicate. This is why starting treatment promptly after symptoms appear can be important, rather than focusing too much on taking a dose at an exact time of day.
Cold sores can have an early stage, often called the prodromal phase, when you may notice tingling, itching or burning before a visible blister develops. Starting treatment as soon as possible after these symptoms appear may give aciclovir the best opportunity to help manage the outbreak. Treatment can still be used after a blister has appeared when it has been prescribed, but it is generally more useful when started early.
Once you have started treatment, follow the dosing schedule on your prescription. Taking doses at evenly spaced times can help you follow the prescribed regimen, and you should continue the treatment for the full course unless your prescriber tells you otherwise.
How Often Is Aciclovir 400mg Taken? Two Different Regimens Explained
This is where aciclovir 400mg can become confusing because the same tablet strength can be prescribed for different purposes. The dose, frequency and treatment length can vary depending on the infection and whether the medicine is being used to treat an active episode or help prevent recurrences.
Treating an active herpes infection
When treating an active herpes infection, aciclovir may be prescribed several times a day for a defined course. The exact regimen depends on the infection being treated. For example, treatment for genital herpes, cold sores and shingles can involve different doses and treatment durations.
Starting treatment promptly may help reduce the duration or severity of symptoms in some situations. Because aciclovir stays in the body for a relatively short time, prescribed treatment schedules usually involve doses spread throughout the day.
Always follow the instructions on your prescription rather than changing the number of tablets or doses yourself.
Preventing frequent recurrences
Aciclovir 400mg can also be prescribed as suppressive treatment for people who experience recurrent herpes outbreaks. Instead of taking it only when an outbreak starts, the medicine is taken regularly over a longer period to help reduce the frequency of recurrences.
Suppressive treatment can reduce the frequency of recurrent outbreaks in some people, but it does not cure herpes. Your prescriber will decide whether this approach is appropriate based on your symptoms, how often you experience outbreaks and your overall health.
Why this matters for you
If you have been prescribed 400mg tablets, your pharmacy label will tell you exactly how many aciclovir tablets to take, how often to take them and how long to continue treatment. That instruction is specific to you and should take priority over a general usual dose guide online.
If your prescription differs from something you have read online, do not assume that either one is wrong. Aciclovir can be prescribed differently depending on the infection being treated, whether you are experiencing a first episode or a recurrence, your kidney function and other individual factors.
Generic information about aciclovir dosage cannot replace the instructions from your prescriber. If you are unsure about your dose or how often to take aciclovir, ask your pharmacist or prescriber before changing your treatment.
| Situation | General pattern (informational only) | Typical duration |
|---|---|---|
| Treating an active cold sore or herpes outbreak | Multiple doses spread across the day | A short, defined course, often around 5 days |
| Preventing frequent recurrences (suppressive therapy) | A lower daily frequency, taken continuously | Months, reviewed periodically by your prescriber |
| Shingles or chickenpox | A higher-strength regimen, usually with 800mg tablets | A longer course, often 7 to 10 days |
This table describes general patterns published in the UK prescribing references, not an instruction for any individual reader. Your prescriber will determine the appropriate treatment, dose and duration for you following an individual clinical assessment.
Why some people ask about 800mg
Aciclovir tablets are available in different strengths, including 800mg tablets, which may be prescribed for conditions that require a different dosing regimen. For example, 800mg tablets are commonly used to treat shingles and chickenpox, which are caused by the varicella zoster virus.
If you have been prescribed 800mg tablets, the regimen will be different from the 400mg or 200mg regimens that may be used for other herpes infections. The frequency and duration of treatment depend on the condition being treated and your individual circumstances. Treatment is generally started as soon as possible after symptoms appear, and some courses may last for 7 to 10 days.
Always follow the instructions on your prescription rather than changing the strength or frequency yourself.
Can You Take Aciclovir on an Empty Stomach?
Yes. Aciclovir tablets can be taken with or without food, so you do not need to eat before taking a dose.
If you notice some stomach discomfort or nausea after taking aciclovir, taking it with a meal or a small amount of food may feel more comfortable. This is not required for the medicine to work, but it can be a practical option if your stomach feels unsettled.
If nausea or stomach pain persists or becomes worse, speak to your pharmacist, GP or prescriber for advice.
It is also important to take each dose with enough fluid and stay well hydrated during treatment, particularly if you have kidney problems.
What to Do If You Have a Missed Dose
If you forget a dose of aciclovir, take it when you remember unless it is nearly time for your next dose. If your next dose is due soon, skip the missed dose and continue with your regular dosing schedule.
Do not take two doses together to make up for a missed dose. If you have missed a dose, simply return to your usual schedule and continue taking the remaining doses as prescribed.
When to seek urgent advice
Serious side effects with aciclovir are rare, but it is important to know what to look out for. Common side effects can include headache, nausea and dizziness.
Contact NHS 111 if you think you may be having a serious side effect, such as signs of an allergic reaction, including swelling of the face or throat, a widespread itchy rash or difficulty breathing. You should also seek advice if you develop symptoms that could indicate liver problems, such as unusual tiredness, itching or yellowing of the eyes, kidney problems such as passing very little or no urine, or blood cell problems such as unusual tiredness, easy bruising or frequent infections.
Aciclovir can also make your skin more sensitive to sunlight. If you are pregnant, think you may be pregnant or are planning a pregnancy, speak to your prescriber or GP before taking aciclovir.
Getting Aciclovir 400mg in the UK
You cannot buy aciclovir tablets over the counter in the UK because they are prescription only medicines, so a clinical assessment is required before they can be supplied. An online consultation can be completed at home and reviewed by a UK registered prescriber, who can determine the appropriate strength, frequency and treatment duration for your situation. If treatment is prescribed, it can be dispensed by a GPhC registered pharmacy.
Aciclovir is available in different formulations, including an oral suspension for people who have difficulty swallowing tablets.
Prices are starting prices and can vary depending on the strength, quantity and treatment duration. The price confirmed during your consultation is the one that applies.
If taking several doses a day may be difficult to fit around your work or daily routine, mention this during your assessment. Valaciclovir is converted into aciclovir in the body and may be prescribed on a different dosing schedule in some circumstances. Your prescriber can advise whether it is an appropriate option for you.
Also tell your prescriber about any prescription medicines, over the counter medicines, herbal remedies or supplements you take. This helps them assess whether the treatment is suitable for you and whether any adjustments or additional advice are needed.
Medical Disclaimer
This article provides general information about how aciclovir is commonly prescribed and does not constitute medical advice. It does not replace advice from a qualified healthcare professional, such as your GP, prescriber or pharmacist, or the instructions on your prescription label and patient information leaflet, which should always take priority.
Aciclovir tablets are prescription only medicines and should be used under the supervision of an appropriately qualified prescriber. The appropriate dose, frequency and treatment duration depend on your individual circumstances and the condition being treated.
Do not change your dose or dosing schedule based on general information. If you are unsure about how to take aciclovir, speak to your pharmacist or prescriber. Do not stop treatment unless a healthcare professional advises you to do so.
