In general, it's not a good idea to give your baby water until he's about 6 months old. Until then, he'll get all the hydration he needs from breast milk or formula, even in hot weather.
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. (While small sips of water probably won't hurt your baby, it's best to check with his doctor beforehand during those first six months.)
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.
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. Carefully follow the package directions for mixing powdered or concentrated formula and don't try to stretch formula by using more than the recommended amount of water.
In some instances â if your baby has gastroenteritis, for example â the doctor might advise you to give him an electrolyte drink like Pedialyte or Infalyte to help prevent dehydration.
Once your baby is 6 months old, it's okay to give him sips of water when he's thirsty. You still don't want to 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.