In general, your baby shouldn’t drink water until he's about 6 months old. Until then, he gets all the hydration he needs from breast milk or formula, even in hot weather.Once your baby is 6 months old, it's okay to give him sips of water when he's thirsty. Don't overdo it, though, or you might give him a tummy ache or make him too full to eat well.After his first birthday, when your baby's eating solids and drinking whole milk, you can let him drink as much water as he likes.
Why it's unsafe to give water to babies 6 months and younger
Giving a baby younger than 6 months old too much water can interfere with his body's ability to absorb the nutrients in breast milk or formula. It can also cause his tummy to feel full, which curbs his desire to feed.
In rare cases, a baby who drinks too much water can develop a condition known as water intoxication, which can cause seizures and even a coma. Water intoxication happens when too much water dilutes the concentration of sodium in the body, upsetting the electrolyte balance and causing tissues to swell.
Is it okay to dilute formula with water?
Don't try to stretch formula by diluting it with water – carefully follow the package directions for making formula and use the recommended amount of water. Adding too much water to your baby's formula not only risks water intoxication, it means that your baby is taking in fewer nutrients than he needs.
What to do if your baby becomes dehydrated
In some instances – if your baby has stomach flu (gastroenteritis), for example – the doctor might advise you to give him an electrolyte drink like Pedialyte or Infalyte to help prevent dehydration.