Short answer: no. You should never reheat a bottle of baby formula – even though it’s frustrating and feels wasteful.
When your baby starts drinking a bottle, their saliva will mix with the formula. This means that the bacteria in their mouth will also end up in the formula, where they can grow and end up making your baby sick. If your baby hasn’t finished a bottle within an hour of starting it, you should throw it out rather than reheating.
Even if your baby hasn’t started drinking their formula, you shouldn’t reheat it once warmed. Warming formula encourages bacteria growth. For up to an hour, this is fine – but after that, it’s possible the bacteria will have reached levels high enough to make your baby sick.
You’ve probably heard that you shouldn't reheat leftover foods that you eat more than once – for babies, you need to be just that little extra bit careful.
How can you avoid wasting formula?
Not being able to reheat your baby’s milk does mean that there will be times when some is wasted. Leftover milk in part-drunk bottles should always be thrown away, as should milk in warmed but untouched bottles.
To cut down on waste, you could try giving your baby cold formula. Not all babies need their formula warmed, and many will be happy to drink it straight from the refrigerator. Giving your baby cold milk cuts down the risk that you’ll warm a bottle that they then don’t drink.
If you find you’re regularly throwing out unused formula, try cutting down the size of your feeds. Your baby might prefer to be fed little and often. If they’re on solid food, it could be that they have already replaced some of their milk calories and don’t need as much formula as they did. Think about having a stash of ready-to-feed bottles in the house just in case – that way you can easily top up a feed that’s too small.