The Independent Pharmacy

Aciclovir Side Effects: Everything You Need To Know Before Using The Genital Herpes Treatment

Andy Boysan
Andy BoysanBPharmDirector & Superintendent Pharmacist

Reviewed on 12 Jul 2024

Seeking treatment for genital herpes can feel embarrassing and distressing, despite this sexually transmitted infection (STI) being extremely common in the UK. Fortunately, effective antiviral medicines like Aciclovir can be used to prevent or treat such infections. But are there any Aciclovir side effects that you need to know about?

If you are considering taking this medication to treat genital herpes, you’ll probably want to know more about the possible side effects of Aciclovir that you may experience during treatment and how likely you are to encounter them.

That’s why we’ve created this guide: to give you an overview of the more common side effects of this medicine, as well as any more serious ones and any possible allergic reactions too. We also cover whether it is safe to take Aciclovir during pregnancy.

For more general information about this treatment, visit our ‘What Is Aciclovir Used For?’ page.

Aciclovir 400mg Tablets
Aciclovir 400mg Tablets
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Summary list of Aciclovir side effects

All medicines have the potential to cause side effects, both mild or severe. However, the benefits of taking these medicines usually outweigh any unwanted side effects.

Aciclovir is considered safe to use as well as being extremely effective. Most people who use Aciclovir will not encounter serious side effects and will generally have an extremely positive experience with this antiviral medicine.

Below is a summary list of the possible Aciclovir side effects that users may experience:

  • Headache
  • Dizziness
  • Nausea
  • Vomiting
  • Diarrhoea
  • Stomach pain or cramps
  • Tiredness
  • Muscle or joint aches or pains
  • Increased sensitivity to light
  • Changes in vision
  • Fluid retention
  • Hair loss
  • Numbness and tingling
  • Shaky/unsteady movement
  • Difficulty controlling movements
  • Extreme confusion
  • Agitation or similar mood changes
  • Hallucinations (seeing or hearing things that aren’t there)
  • Itching
  • Skin rash or hives (or other signs of an allergic reaction)
  • Fever
  • Trouble speaking
  • Yellowing of the skin or eyes (jaundice)
  • Kidney pain (felt in the back)
  • Unusual bleeding (such as excessive nosebleeds) or bruising
  • Seizures
  • Loss of consciousness
  • Swelling of face, lips, or tongue
  • Difficulty breathing
  • Decreased urine output or blood in the urine

As you can see, the possible side effects of Aciclovir range from the more mild and common side effects (such as nausea or headaches) to very rare and more serious side effects (such as seizures or signs of kidney problems).

The likelihood of experiencing any of these side effects will vary from user to user; what one individual may encounter while taking any medication can be completely different from another.

This is not a definitive list; if you are concerned that you are experiencing an Aciclovir side effect that we have not mentioned above, speak to your GP.

Similarly, if you find that you are experiencing any severe, persistent or painful side effects due to taking Aciclovir, then you should consult your doctor for further guidance straight away.

More common side effects and symptoms

Many of the Aciclovir side effects we’ve listed above will not be experienced by the majority of users when used to prevent or treat genital herpes infections.

However, there are some more common side effects of Aciclovir tablets that are generally more likely to occur. These common side effects may affect more than one in 10 people who take Aciclovir tablets.

The most common Aciclovir side effects include:

  • Dizziness
  • Headache
  • Nausea (feeling sick)
  • Vomiting (being sick)
  • Diarrhoea
  • Stomach pain
  • Skin rashes
  • Itching
  • Tiredness
  • Fever
  • Increased sensitivity to sunlight or artificial light such as sunbeds (in either the eyes or skin)

These more common side effects do not normally need medical attention and it is usually recommended that you keep taking the medicine until you have completed your course of treatment (if it’s your first time taking Aciclovir, your first course will probably last for around 10 days).

However, if these side effects bother you, are painful, or don’t go away, you should speak to your GP or pharmacist.

Serious side effects and symptoms

Although unusual, serious Aciclovir tablets side effects can occur.

Below, we have listed some of the more unlikely but serious Aciclovir side effects and symptoms:

  • Mental/mood changes (such as confusion or agitation)
  • Unsteadiness (shaky/unsteady movement or difficulty controlling movements)
  • Numbness or tingling in your hands or feet
  • Changes in vision (blurred or double vision, extreme light sensitivity)
  • Hallucinations (seeing or hearing things that aren’t there)
  • Severe rash, hives, or a rash that causes blisters and peeling
  • Fever
  • Seizures
  • Trouble speaking
  • Yellow skin or the whites of your eyes turning yellow (this could be indicative of liver or gallbladder problems)
  • Signs of kidney problems (such as a change in the amount of urine, blood in the urine, unusual back/side pain)
  • Unexpected bleeding or infections such as mouth ulcers, bruising or bleeding gums (which may be indicative of a serious blood problem)
  • Difficulty breathing

It is extremely unlikely that you will experience any of the above serious side effects. However, if you notice any of these signs or symptoms, you should stop taking Aciclovir and seek medical attention immediately.

Similarly, if you feel like you are experiencing symptoms that are dangerous or life-threatening — even if they are not on this list — then seek medical help straight away.

There are multiple different strength doses of Aciclovir that are used to treat a range of infections and conditions. Taking an Aciclovir dosage that are too strong for your condition or the infection that you’re trying to treat may result in an increased chance of unwanted side effects. You should always take this medicine — and any other — exactly as your doctor or pharmacist tells you, or follow the directions on the patient information leaflet.

Allergic reactions

A serious allergic reaction (anaphylaxis) to Aciclovir tablets is very rare, but it can happen.

You should seek immediate medical attention if you notice any of the below symptoms and signs of a serious allergic reaction:

  • Any skin rashes (including itchy, red, swollen, blistered or peeling skin)
  • Tightness in the chest or throat
  • You have trouble breathing (such as breathlessness or wheezing)
  • Swelling (particularly of the face, mouth, lips, tongue or throat)
  • Trouble talking
  • Severe dizziness

These signs indicate a serious allergic reaction to Aciclovir, and you should treat this as an emergency. Get emergency medical attention immediately by calling 999 or going to A&E.

Aciclovir tablets and pregnancy: are they safe to use?

It’s thought that most herpes medications — including Aciclovir — are relatively safe to take while pregnant, at least in the early stages of pregnancy.

There is currently no clinical evidence to suggest that Aciclovir tablets are associated with an increased risk of birth defects. Existing studies of women taking Aciclovir in the first trimester of pregnancy indicate that there is no higher chance of defects in babies exposed to Aciclovir.

Similarly, no links between Aciclovir and problems with the pregnancy — including miscarriage, stillbirth, preterm birth or low birth weight — have been found.

Treating genital herpes with Aciclovir in pregnancy

If you want to treat genital herpes while you are pregnant, you should speak to your doctor.

When you are considering taking Aciclovir in pregnancy, it is important to weigh up the health benefits of taking it against any possible risks to you or your baby. This can vary depending on how many weeks pregnant you are.

Your doctor will discuss your options and any potential risks with you, and help you decide what is right for you and your baby.

Use of Aciclovir in pregnancy just before birth

If you have been infected with the herpes virus and you experience recurrent outbreaks of genital herpes, your doctor or consultant may suggest using Aciclovir in tablet form in the four weeks prior to delivery to prevent the recurrence of genital herpes.

This is so that you don’t pass on the viral infection to your baby during delivery.

What if the father takes Aciclovir?

There is currently no specific research into the potential risk of birth defects or harm to the baby if you are a man using Aciclovir to treat genital herpes before or around conception.

However, it is thought to be very unlikely that Aciclovir taken by the father will have any negative effect on the unborn child or increase risks to a pregnancy.

Aciclovir and breastfeeding

It's generally considered safe to breastfeed while taking Aciclovir.

If you're taking Aciclovir in the form of tablets or liquid, then some of the antiviral medicine will pass into your breast milk. However, this is in extremely small amounts and is very unlikely to cause harm to your baby.

If you are thinking about using Aciclovir while breastfeeding, it is best to speak to your doctor. They can advise you on what is best for you and your baby.


Aciclovir is an effective and safe antiviral medicine that is used to prevent or treat viral infections from the herpes simplex virus (HSV), such as genital herpes.

Most people who use Aciclovir will not experience any negative side effects at all and will have a very positive experience with this antiviral medicine.

More common side effects of Aciclovir (affecting over one in 10 users) include nausea, vomiting, diarrhoea, and sometimes tiredness (if you are on a higher dose). These more common Aciclovir side effects do not usually need medical attention and it is usually recommended that you complete your course of treatment.

If you encounter any severe or worrying Aciclovir tablets side effects, you should contact your doctor or pharmacist for advice on how to proceed.

You can buy Aciclovir 400 mg tablets for genital herpes at The Independent Pharmacy if you think this might be the right treatment for you. You will need to complete a free online consultation, answering a few quick questions first so that we can establish if this is the most suitable treatment for you. Alternatively if you need Aciclovir tablets for cold sores, we are also able to provide them online.

Once your form has been approved by one of our qualified prescribers, we can issue your medication. We offer a fast, secure and discreet online service and delivery straight to your door — order before 4 pm and receive your treatment the next day.


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