I switched my daughter to a Medium flow nipple when she moved from eating 4 oz to 5 oz (which was probabaly when she was just shy of four months old). You might want to try those first before going all the way to the fast flow. She's been on these for about a month, but the first two weeks she was making such a mess getting used to them...I can't even imagine what would have happened if we went right to the fast flows.
We moved up to the level 2, or 'medium', flow nipples a couple weeks ago. It makes a world of difference! She finishes her bottle w/ out falling asleep and we don't have any 'snacking' going on b/c she gets tired of sucking, then is ready again in 30 min.
Re: Fast-flow nipple?