I breastfed for 4 months but did give some formula when needed in there. Also just switched to formula with no problems as he preferred the bottle any way. I pumped for awhile after I went back to work but it got to be a pain as he wouldn't nurse at night either so I just switched to formula. We used the similac that came from the hospital & then the enfamil that I got in the mail. He seemed a little gassy so we switched to the enfamil gentlease & that worked great. So then we used the target & walmart brands of the gentle & they are all the same. He's eating table food now so we've switched to milk no problems. They grow up quick!