I know the question was about azathioprine, but you may be interested to
find a short paper titled "Successful breast feeding while mother was taking
cyclosporin" in the British Medical Journal, 23 Aug 1997, Vol 315: 463.
Cyclosprorin has similiar indications to azathioprine (although it is
obviously a different drug). They say " A mother receiving drug treatment
who has ben counselled against breast feeding will reasonably assume that
the hazards of a drug have been balanced against the disadvantages of
formula feeding. This is not so for cyclosporin".
Yeah!

Lisa Amir
GP / IBCLC in Melbourne, Australia