I have had a couple of mothers nurse on mercaptopurine without apparent
problems to the baby.  This is not proof, but it may be somewhat reassuring.

The fact that a drug that is safe during pregnancy is necessarily safe
during breastfeeding is not actually true, not as a general rule, since the
mother will metabolize the drug for the fetus.  But with a drug like
mercaptopurine, I believe this is true.  However, is mercaptopurine safe
during pregnancy?

Hale says there is no data available on excretion into milk.  Wouldn't this
be a nice little paper, to do levels on the milk?

I lean to saying yes to breastfeeding, doing milk levels and watching the
baby closely.

Jack Newman MD, FRCPC