Recently managed to acquire some authoritative advice on immunosuppressant drugs and breastfeeding -- I know some of you are health professionals who might be interested, so here goes.
The drug in question is azathioprine, which I've been taking for 20 years. Official advice is that it is contraindicated in breastfeeding (and in pregnancy, but that's another story) since it could theoretically pass into b-milk. However, if you look up medline, there is very little actual information, and what studies there are involve about 5 patients.
After 3 children and varied contradictory advice, I have been told by a very senior consultant with extensive experience of prescribing this drug to women of child-bearing age, that in his opinion the level of drug passing through to the milk is very low, and that the benefits of feeding outweigh the risks. Apparently this is also the case for oral steroids (prednisolone).
FWIW all my babies were fully b-fed and seem to have immune systems like armoured tanks!
Hope this information is of use to someone...