Formula that you mixed must be thrown out after 24 hours. If your baby does not drink all the formula within one hour, throw it out. Do not refrigerate a leftover bottle with formula with the intention of your baby drinkig it later because bacteria could make your baby sick.
Beside this, is it safe to mix breastmilk and formula in the same bottle?
While there's nothing wrong with mixing breast milk and formula in the same container, it's not recommended simply because you don't want to waste a single drop of your precious breast milk. Formula from a bottle that your baby has drunk from must be discarded within an hour of preparation.
Is it OK to mix breast milk and formula together?
While some women who have sore nipples may decide to pump exclusively for a while, this strategy could cause your supply to dwindle further. If you need to supplement your infant's diet with pumped breast milk and formula, they may be mixed together safely in the same bottle.