I dug around on this for a mom a few years ago and ended up writing to
someone in Belgium, who had information on the only case I could find
written up at that time (1993).  Among his comments in answer to my
questions:
Milk peak concentration, following a 100 mg oral dose was 5.0 ng/ml.  Time
after which it was no longer detectable was 13.25 hr. (Lower limit of
detection was 1.0 ng/ml.)  "High levels of mebendazole in the breast milk
are very unlikely since absorption of mebendazole is limited."  And - get
this - "mebendazole is used in European dairy cattle."

There's also a write-up in JHL '94 10(4):269.  One author felt it could
decrease milk supply.  My Belgian contact did not agree.  Neither, I
imagine, do European cows.

Diane Wiessinger, MS, IBCLC, LLLL  Ithaca, NY
(new address:  [log in to unmask])