Something that my GP told me is that the drug companies themselves don't get accurate results on BFing cause it's difficult to measure effects on mice in that respect (his exact words) so the drug companies tend to say "not recommended while BFing" as a blanket cause. If you look up MERKS (I think?) then it says what's on the info inside the packet only. So my GP has said to me with my migraine tabs that is says no, but it's OK.
I guess that if the GP takes what the drug companies have as their info, then they will say 'no' to a lot of drugs. If you're ever worried, give Rodney @ Monash Medical a call - he's made BFing a speciality of his and is absolutely wonderful.