In the first place, women who are breastfeeding should not be taking
oestrogen containing birth control pills or any other pills containing
oestrogens, exactly because they may significantly decrease the milk
supply.

I think that fluconazole can decrease the milk supply even without the
concomitant use of oestrogens.  Its parent, ketoconazole, is known to
have androgen like effects, or stimulate androgens, which might explain
this.  Usually it is not a problem, and usually, even if it is, the milk
comes back pretty smartly after the course of treatment.

Jack Newman, MD, FRCPC