evidence: Understanding why babies shouldn't drink water under 6 months old It is advised that you shouldn’t give your baby water before they are six months old. Before then, they get all the nutrients they need from breast milk and do not need extra fluids, even in hot weather. [...] Giving water to babies younger than six months puts them at risk of infection. It may also cause your baby to drink less breast milk and miss out on the nutrition they need. In some severe cases, when a large amount of water is consumed, a condition known as water intoxication can occur. This is when sodium in the baby’s bloodstream becomes diluted, causing tissues to swell and leading to dangerous interference with brain activity.