lactifuge

from WordNet (r) 3.0 (2006)

lactifuge
    n 1: any agent that reduces milk secretion (as given to a woman
         who is not breast feeding)

from The Collaborative International Dictionary of English v.0.48

Lactifuge \Lac"ti*fuge\, n. [L. lac, lactis, milk + fugare to
   expel.] (Med.)
   A medicine to check the secretion of milk, or to dispel a
   supposed accumulation of milk in any part of the body.
   [1913 Webster]