I do formula and breast milk and my lil man is 3 months.

I've been do this for nearly 3wks doing breast milk then formula only because she has troubles nursing and I just stopped with the breastmilk cause it was doing more harm than good with her because it was constipating her horribly doing both were she would struggle all day and night hurting...since stopping it I also switched her milk from Similac (harsh harsh on my kids) switched to Gerber Good Start Gentle and Mommys Bliss probiotics and shes done sooo much better now.

They say breast first then I guess if you couldn't produce enough supplement with formula