To : Janaki Costello
Re : Fluorouracil

Whereas fluorouracil when administered IV is quite toxic, when applied
topically it is very poorly absorbed (less than 6%).  Oral absorption is poor
and extremely erratic ( 0 to 80%) so it is never used orally.  The recommended
IV dose is 12mg/kg/day for 4 days,  so the amount you use topically would be
miniscule compared to the IV dose.

Its volume of distribution is actually low,  only penetrating 22 % of body
water, so it would be rapidly cleared, although it has several inactive and
nontoxic metabolites that have much longer half-lives.  The half-life of
fluorouracil is only 8-22 minutes, although it is sequestered in selected
tissues.

I would seriously doubt that the amount absorbed via topical application would
be any contraindication to breastfeeding.

Regards
Tom Hale, Ph.D.