I reused once with the 4oz liners when LO was transitioning from 4 oz to 5oz at a feeding (we hadn't bought 8oz bottles yet), so I mixed up another ounce with the same liner I'd just used. I've never heated his bottles, so I can't speak to how they'd react if you reheated with the same liner. However, I found that after you've used the liner it tends to collapse a bit at the top, which would lead to leaks if you reused. Like I said, I only did it once to just make one more ounce and so as not to waste a liner. IMO, though, it's not really worth reusing a liner. All sanitary issues relating to reusing liners aside, one of the benefits of liners is not having to constantly clean bottles; if you're having to clean the liners, what's the point?