Hi Laurie
I have an article from the Am J Obstst Gynecol March 1988 by Meyer et al which
describes one study on a woman with a 1 year old who was treated for shingles
(herpes zoster) with acyclovir 200mg 5 times a day - the authors found milk
concentrations averaged 3.24 fold higher than serum levels which 'suggest
a low infant dose'. The half life in milk was demonstrated as 2.8 hours.
They concluded that the dose the infant would receive would be very low and
that the use of oral acyclovir in nursing mothers is of low theoretical
risk as long as the infant has normal renal function. As your client
is being treated for herpes simplex, rather than zoster, the dose she will
be given is less than 200mg 5 times a day and I assume the amount in breastmilk
would be low. Hope this helps :)

Fiona Coombes
Family Physician, IBCLC, NMAA Breastfeeding Counsellor
Perth Western Australia