Is water safe for a baby under 6 months?

The verdict

No, not as a drink, and this is one of the few infant feeding rules with a mechanism that can hurt a baby within hours rather than months.

Until around six months, breast milk or first infant formula is the whole diet, water included. The NHS states that breast milk is the only food or drink babies need in the first 6 months of their life, and that fully breastfed babies do not need any water until they've started eating solid foods. The World Health Organization phrases its exclusive breastfeeding guidance as not giving babies additional food or drink, even water, unless medically necessary.

There is one small carve-out and it is British. The NHS says formula fed babies under 6 months of age may need small sips of cooled boiled water during hot weather as well as their usual milk feeds. Sips, in hot weather, in addition to milk, never instead of it.

Why extra water is dangerous, not just useless

An infant's kidneys cannot excrete a water load the way an adult's can, and an infant's total body water is small. Give enough plain water and the sodium concentration in the blood falls. The AAP describes the consequence directly: extra water disturbs electrolyte and mineral balances such as calcium, sodium and potassium, and this can lead to major health problems, including seizures. The condition has a name, water intoxication, and hyponatraemic seizures in young infants are a recognised emergency presentation.

The second harm is nutritional and quieter. Water fills a stomach that holds very little. The AAP notes that adding extra water to formula or giving juices reduces the amount of nutrients the baby will receive, and that this can slow growth and development. A baby who takes 60 ml, about 2 fluid ounces, of water takes 60 ml less milk, and there is no slack in the system to absorb that.

Never stretch formula

This is the most important sentence on this page. The AAP flags it explicitly: news reports have found parents diluting formula to try and save money or feeding water in addition to breast milk or formula, and this can lead to water intoxication. Formula that is diluted with too much water can cause serious health and development problems.

Always make formula to the scoop ratio on the tin. If you cannot afford formula, that is a problem for your health visitor, GP, midwife or a local food bank, not one to solve with the kettle. In the UK, ask about Healthy Start. Diluting formula turns a money problem into a medical one.

What about the water in the formula itself

That is a separate question with its own rules, and here the countries diverge sharply.

The NHS treats every powdered formula feed as potentially contaminated, because infant formula powder is not a sterile product. Its method: fill the kettle with at least 1 litre of fresh tap water, boil it, and leave the water to cool for no more than 30 minutes so that it remains at a temperature of at least 70C, which is 158F. Water at 70C or more will kill any harmful bacteria in the powder. Do not use artificially softened water or water that has been boiled before.

The AAP is much less prescriptive. Its instruction is to boil the water when needed, specifically for infants younger than 2 months of age, those who were born prematurely and those who have a weakened immune system, then let it cool for about 5 minutes before mixing. For a healthy term baby over two months, US guidance does not require boiled water at all.

Both positions are current, and a British parent following AAP advice would be under-treating the powder by NHS standards.

Bottled water

The NHS says bottled water is not usually recommended for making up infant formula feeds, because it's not sterile and may contain too much salt (sodium) or sulphate. It permits it in rare circumstances, such as when drinking water is unavailable or has been contaminated, and still requires the full 70C method.

What changes at around six months

Everything gets easier. The NHS advice once solid food starts is to introduce your baby to drinking from a cup or beaker from around 6 months and offer sips of water with meals, and it adds the point most parents miss: drinking water for babies over 6 months does not need to be boiled first. Cold tap water is fine.

Health Canada's advice at this stage is that older infants can be offered water from an open cup along with complementary feedings, with the parent holding the cup against the mouth at first, and that older infants can pace their own intake, which makes it easier for them to control their breathing and swallow when they are ready.

Use an open cup or a free-flow cup without a valve rather than a bottle or teat. The NHS reason is dental and developmental: it helps your baby learn to sip and is better for your baby's teeth. The NHS also says that when using a bottle or trainer cup, do not put anything in it other than breast milk, formula milk or water, and do not add anything else, including sugar, cereals, baby rice or chocolate powder.

Milk remains the main drink. NHS hot-weather advice from around six months is to offer sips of water from a cup with meals, remembering that breast milk or infant formula should be their main drinks during the first year, with some additional water outside mealtimes in hot weather.

Hot weather without water

If your under-six-month baby seems thirsty in a heatwave, the answer is more milk, not water. The NHS notes that during hot weather breastfed babies may want to breastfeed more than usual. Feeding more often does the job water would do, without the sodium risk.

Other things that help: keep the room cool by closing blinds or curtains during the day, use a fan to circulate air but do not aim it directly at your baby, attach a parasol or sunshade to the pushchair, and never cover a pram with a blanket, because that leads to overheating.

Signs a baby is genuinely dehydrated and needs to be seen: fewer wet nappies than usual, a sunken soft spot, a dry mouth, unusual drowsiness or floppiness, or no tears when crying. That is a call to your GP, health visitor or NHS 111, not a reason to give water.

Where guidance differs

The UK allows sips; the WHO and the AAP do not. The NHS explicitly permits small sips of cooled boiled water in hot weather for formula-fed babies under six months. WHO guidance says no additional food or drink, even water, unless medically necessary. The AAP says babies in the first 6 months after birth do not need water or other liquids such as juices in addition to formula or breast milk, unless a pediatrician specifically advises otherwise. All three would agree the volume in question is trivial; the framing differs.

Formula preparation diverges much more sharply. The UK requires water at 70C (158F) or above for every feed. The US requires boiled water only for babies under two months, preterm babies and immunocompromised babies. That is a real, current difference in the level of protection taken against bacteria in the powder.

Boiling drinking water stops at six months in the UK. The NHS is clear that water for babies over 6 months does not need to be boiled. Under six months, the water for those hot-weather sips does.