Antiviral medicines are used to treat viral infections by interfering with viral replication, but their side effects and precautions vary depending on the specific medicine. This guide focuses on aciclovir and valaciclovir, which are used to treat herpes simplex infections and shingles.
The most common problems are headache, nausea, dizziness, stomach upset, and mild local irritation from cream, while serious side effects are rare and can include kidney problems, severe allergic reactions, and liver or blood cell issues. Antivirals target viruses, whereas antibiotics are used for bacterial infections and do not treat the flu.
"Antiviral" covers a wide range of medicines used against different viruses, and their side-effect profiles are not interchangeable. This guide focuses specifically on aciclovir and valaciclovir for cold sores and related herpes infections. If you have been prescribed one of these medicines in the UK, this guide explains the common and serious side effects, drug interactions, kidney-function precautions, pregnancy and breastfeeding considerations, age-related precautions, differences between cream and tablets, and what to do about missed doses or accidental overdose. If you have been prescribed a different antiviral for another condition, its safety information may be different.
If you are an adult in the UK who has been prescribed one of these drugs and want clear safety information before or during treatment, this guide explains the common and serious side effects, drug interactions, kidney function precautions, pregnancy and breastfeeding advice, age-related dosing, when cream differs from tablets, and what to do about missed doses or overdose. If you have been prescribed a different antiviral for a different condition, its safety information will not necessarily be the same as what is described here.
Aciclovir and valaciclovir are generally considered well-tolerated medicines with a long history of use, but understanding their risks matters if you have kidney problems, take other medicines, are older, or are pregnant or breastfeeding.
Common Side Effects of Antiviral Treatment
Most people tolerate aciclovir and valaciclovir well. Side effects, when they occur, are usually mild and short-lived.
| Formulation | Reported side effects |
|---|---|
| Aciclovir or valaciclovir tablets | Reported effects include headache, dizziness, nausea or vomiting, diarrhoea, stomach discomfort, rash, itching and tiredness. The frequency varies by medicine and formulation. |
| Aciclovir 5% cream | Mild, transient burning or stinging at the application site can occur, this is classified as uncommon. |
This is a general overview rather than an exhaustive list. The patient information leaflet supplied with your specific product contains the full list of reported adverse effects and their frequencies, which can vary between medicines and formulations. If side effects are troublesome or persistent, speak to a pharmacist or prescriber rather than stopping treatment without advice.
Serious Side Effects: What to Watch For
Serious reactions are rare, but it is useful to know which symptoms require urgent medical attention.
- Signs of a severe allergic reaction (anaphylaxis): swelling of the face, lips, tongue or throat, difficulty breathing, or a severe rash. This needs emergency medical attention.
- Signs of a liver problem: unusual tiredness, itchy skin, yellowing of the skin or eyes (jaundice).
- Signs of a kidney problem: passing little or no urine, or a noticeable change in how much you are passing.
- Signs of a blood cell problem: unusual tiredness, bruising more easily than normal, or frequent infections.
- Confusion, agitation, drowsiness or hallucinations have occasionally been reported, more often in people with reduced kidney function or those receiving aciclovir intravenously in hospital, rather than with standard oral tablet courses for cold sores.
If you notice any of these symptoms, seek medical advice promptly. Symptoms of a severe allergic reaction require emergency medical attention.
Drug Interactions: What Genuinely Matters
Aciclovir has relatively few recognised interactions, but some medicines can affect its clearance through the kidneys. The key interactions highlighted by the NHS are listed below.
| Medicine | What is known |
|---|---|
| Probenecid (used for gout) | Can increase aciclovir levels in the blood, since both are cleared by the kidneys via a shared pathway |
| Cimetidine (used for stomach ulcers or reflux) | Can also increase aciclovir levels, for the same reason |
| Mycophenolate mofetil (used after organ transplant) | Levels of both mycophenolate mofetil and aciclovir may be increased when taken together, again due to shared renal clearance |
These are the three medicines specifically named by the NHS as having recognised interactions with aciclovir tablets. All three involve renal clearance, which is relevant because aciclovir is cleared from the body largely unchanged through the kidneys. Other medicine-related precautions may apply depending on the formulation and clinical circumstances.
No clinically significant medicine interactions have been identified with aciclovir cream, which results in minimal systemic absorption.
Other medicines that can affect kidney function may also require consideration, particularly with higher-dose or intravenous aciclovir. If you take an immunosuppressant or any medicine that may affect kidney function, tell your healthcare professional so that potential risks can be assessed. Also mention any over-the-counter medicines and supplements you take.
The NHS does not list an interaction between aciclovir and alcohol, and food and drink can generally be taken as usual. If you take other medicines, including antihistamines or painkillers, check with a pharmacist or prescriber if you are unsure about combining them with your antiviral. Regular or high-dose NSAID use, existing kidney impairment, older age or dehydration may increase kidney-related concerns, so mention regular ibuprofen or other NSAID use to your healthcare professional if this applies to you. Take the medicine exactly as prescribed and do not change the dose or duration without medical advice.
Kidney Disease and Function: An Important Safety Consideration
Aciclovir has limited solubility in urine. If you become dehydrated, urine becomes more concentrated, which can allow aciclovir to crystallise within the kidney tubules. This kidney toxicity is a form of nephrotoxicity associated with aciclovir. This is the mechanism behind the NHS advice to drink plenty of water while taking aciclovir tablets, particularly if you have any existing kidney problems. There is no official specific volume or number-of-glasses figure. This is why the NHS advises drinking plenty of water while taking aciclovir tablets, particularly if you have kidney problems.
Because aciclovir can lead to nephrotoxicity, people with kidney disease may need a lower dose or adjusted frequency. This is a decision made by a prescriber based on your individual kidney function, not something to calculate or adjust yourself. If you have a known kidney condition, mention it clearly during your consultation.
Pregnancy and Breastfeeding
Aciclovir and valaciclovir may be used during pregnancy where clinically appropriate, based on an individual assessment. UK pregnancy safety data have not shown an increased risk of birth defects associated with aciclovir use above the background rate seen in pregnancy. Aciclovir is also generally considered compatible with breastfeeding, since only a small proportion of a maternal dose passes into breast milk. Always tell your midwife, GP or pharmacist that you are pregnant or breastfeeding before starting treatment, so your individual circumstances can be considered.
Age-Related Considerations
Kidney function naturally tends to decline with age, which is why older adults may be more likely to need a dose adjustment. This is assessed individually rather than applied as a blanket rule based on age alone. In children, cream is generally suitable for cold sores, while tablets may be prescribed for conditions such as chickenpox or shingles, with dosing calculated according to the child's individual circumstances by a prescriber. Detailed paediatric dosing is outside the scope of this consumer guide and is a matter for your child's prescriber.
Other Situations Worth Mentioning at Consultation
- A previous allergic reaction to aciclovir or valaciclovir, which is a contraindication to further use.
- Any known kidney or liver condition.
- Aciclovir tablets and valaciclovir are prescription medicines in the UK and should be taken only as directed by a healthcare professional.
- A weakened immune system, whether due to a medical condition or treatment such as chemotherapy, especially if you are at high risk of complications or have other health problems.
- Any rare hereditary condition affecting galactose or lactose tolerance, since some tablet formulations contain lactose as an inactive ingredient. Check the patient information leaflet for your specific product if this applies to you.
Missed Doses and Accidental Overdose
If you miss a dose, take it as soon as you remember unless it is nearly time for your next dose, in which case skip the missed dose rather than taking two doses together. If you accidentally take more than prescribed, seek advice from a pharmacist, doctor or NHS 111, particularly if you feel unwell or have kidney problems.
Aciclovir Cream vs Tablets: Different Safety Profiles
It is worth being clear that cream and tablets are not interchangeable in terms of what they are for, or their safety considerations. Cream is applied to the lips and surrounding skin only, is not intended for use inside the nose or mouth or near the eyes, results in minimal absorption into the bloodstream, and has no clinically significant medicine interactions identified. Tablets are absorbed systemically, are cleared by the kidneys, and are therefore the formulation for which the interaction and kidney-related precautions covered in this guide are most relevant.
Medical Disclaimer
This guide is provided for general information only and does not constitute medical advice. It is not a substitute for consultation with a qualified healthcare professional. Aciclovir and valaciclovir are prescription medicines in the UK and should only be used as directed by an appropriately qualified healthcare professional. This article provides general information about potential side effects, interactions and precautions but does not provide individual treatment or dosing advice. Suitability depends on factors such as your medical history, kidney function, other medicines you take, and whether you are pregnant or breastfeeding. Do not start, stop or change treatment without speaking to your prescriber or pharmacist first. If you develop symptoms of a severe allergic reaction, severe confusion or other serious or rapidly worsening symptoms, seek urgent medical attention.
