G.B.
Well, it's stronger in the hind milk than when first feeding. I've also heard of mom's taking it pumping about 8 hours after then take it then discarding that milk.
From what I've seen, which hasn't been many, it doesn't effect the amount of milk it just makes managing how you manage your body and the milk in it.
The baby does get some of the drug, but again, the hind milk has much more of the drug in it and about 8-10 hours after taking it that's when it's strongest in the milk.