We use Dr. Browns bottles, and a week or two before DD was 3mo she was taking FOREVER to finish her bottles, and I went up on the nipple size - to a level 2 and it fixed it. I just bought one pack of 3 of the size 2's and started testing those - her morning bottle is always her slowest, which never made sense because it had been since 11 the night before when she was DF and you think she'd been starving, but no. So I used them for her morning bottle and she did great. There was some leakage and sputtering for a day or two, but she got used to the faster flow and was MUCH more interesting in eating again. HTH