All of the articles I have are dated pre-2000.  I think it has been pretty well documented that regular use of a formula supplementary feeding reduces mother's milk supply and leads to shorter duration of breastfeeding.  Using a formula supplement instead of a feeding or to top up  feeding tells the breast, who tells the brain, we don't need to produce as much milk.  This is the beginning of a downward slope that leads to more formula, less time at breast and eventual cessation of lactation.  



The only post 2000 research I found -  Limited amount of formula may facilitate breastfeeding: randomized, controlled trial to compare standard clinical practice versus limited supplemental feeding, Stranak et al, 2016 in PLOS ONE DOI:10.1371/journal.pone.0150053 Feb 26, 2016.  Their study said controlled formula use did not have an adverse effect on rates of breastfeeding in the short  and long term. [!]  interesting when all the earlier studies say that formula use led to shorter duration of breastfeeding.
They only checked duration at 3 & 6 mo. on 104 infants in trial group and control group.Not sure how long is pp stay.



Another valid source about supplementation with formula is the 2017 revision of the ABM protocol #3, available at their web site                                     																																											
 

 Sonya's request
2) The risks of decreased milk supply with formula supplementation (not the risks of decreased exclusivity and duration of breastfeeding, or perceived decreased supply, but of actual decreased milk supply)





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