Hi thr, my son has been using frisolac AR since he was born. Because AR is a thicker formula since its purpose is to relief constipation for babies. I used dr brown's bottle too unfortunately the milk didnt flow out and my baby was really fussy back then. Its either you change bottle or you poke the teat to be a little bigger. You can do that if baby is 2mth plus, if its newborn i suggest u change milk bottle better