Aciclovir or valaciclovir can be used during breastfeeding. Recommendations apply to full term and healthy infants.

General considerations

It is important to complete an individual risk assessment for your patient and to apply the principles of prescribing in breastfeeding when looking at the available information and making treatment decisions.

This article includes the commonly used antiviral medicines for treating herpes simplex and varicella zoster viral infections such as cold sores, chicken pox, shingles and herpetic genital or eye infections.

Advice is also available on managing viral infections Covid-19 and influenza during breastfeeding.  Some other viral infections, for example HIV, viral hepatitis or cytomegalovirus require highly specialist advice. Contact our specialist service for further advice if the specific infection or antiviral medicine isn’t included here.


Aciclovir is the preferred antiviral for treating herpes simplex and varicella zoster as there are only very small amounts present in breast milk.

Valaciclovir is also an acceptable alternative, since it is converted to aciclovir before passing into breast milk.

Ophthalmic preparations have limited systemic absorption in the mother, and pose negligible risk to a breastfed infant.

Treatment choice should be primarily based on clinical indications, with suitability in breastfeeding as a secondary consideration.

Clinical considerations

The herpes virus cannot be passed on through breastmilk. However, there is a risk of chicken pox to the infant if they come in contact with the shingles sores.  Therefore, if sores are on the breast or close to the nipple, it may not be possible to feed from that breast, although this would need thorough assessment. Infant feeding support should also be sought in order to effectively maintain milk supply.

Specific recommendations

Patient Information

The NHS website provides advice for patients on the use of specific medicines in breastfeeding.

Contact us

Get in touch with the UK Drugs In Lactation Advisory Service (UKDILAS), our specialist breastfeeding medicines advice service if you need support in the following situations:

  • you need further advice
  • the medicine in question is not included here
  • the infant is unwell or premature
  • multiple medicines are being taken

About our recommendations

Recommendations are based on published evidence where available. However, evidence is generally very poor and limited, and can require professional interpretation. Assessments are often based on reviewing case reports which can be conflicting and lack detail.

If there is no published clinical evidence, assessments are based on: pharmacodynamic and pharmacokinetic principles, extrapolation from similar drugs, risk assessment of normal clinical use, expert advice, and unpublished data. Simulated data is now increasingly being used due to the ethical difficulties around gathering good quality evidence in this area.


Full referencing is available on request

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