If a baby under 6 months drinks a small amount of water, nothing harmful will likely happen. But giving water regularly or in larger quantities to a young infant can be dangerous, potentially causing a condition called water intoxication, where sodium levels in the blood drop low enough to trigger seizures. The American Academy of Pediatrics recommends breast milk or formula as the sole source of nutrition and hydration for roughly the first 6 months of life.
Why Water Is Risky for Young Babies
A newborn’s kidneys are far less efficient at processing water than an older child’s or adult’s. An adult kidney can concentrate urine to about 1,300 mOsm/L (a measure of filtering strength), while a full-term newborn tops out around 700 mOsm/L. That means a baby’s kidneys simply can’t flush excess water fast enough. When too much plain water enters the body, it dilutes the sodium in the bloodstream, a condition called hyponatremia.
According to the CDC, symptoms begin when sodium levels drop rapidly and total body water increases by roughly 7% to 8% or more. In a baby who weighs 10 pounds, that’s a surprisingly small volume of water. The low sodium disrupts normal cell function in the brain, causing cells to swell. In mild cases, this leads to irritability and drowsiness. In severe cases, it can progress to seizures, loss of consciousness, or worse.
Signs of Water Intoxication in Babies
Babies can’t tell you they feel off, so the warning signs are behavioral and physical. Watch for unusual drowsiness or difficulty waking, irritability that seems different from normal fussiness, and a puffy or bloated-looking face or belly. Nausea, vomiting, and swelling in the hands or feet are also red flags. Muscle weakness or twitching can appear as the condition worsens.
If a baby has a seizure or becomes unresponsive after ingesting water, that is a medical emergency. The faster sodium levels are corrected, the better the outcome.
How Water Affects Feeding and Growth
Even if water doesn’t cause intoxication, it can still harm a baby by filling up their tiny stomach. Babies have limited appetite, and water has zero calories and no nutrients. Every ounce of water a baby drinks is an ounce of breast milk or formula they won’t want. Over time, this pattern leads to inadequate calorie and nutrient intake, slow weight gain, and poor growth.
The same risk applies to diluted formula. Adding extra water beyond the manufacturer’s instructions stretches the formula further but reduces the concentration of calories, protein, fats, and minerals per feeding. Oregon’s Health Authority notes that over-dilution of formula is a recognized cause of failure to thrive in infants. Always follow the exact water-to-powder ratio on the label.
What About Hot Weather?
Parents often worry their baby will get dehydrated in summer heat and wonder if water is needed. For breastfed babies under 6 months, the answer is no. Breast milk is roughly 88% water and adjusts in composition based on the baby’s needs. During hot weather, babies tend to nurse more frequently, which keeps them hydrated. The NHS confirms that fully breastfed babies do not need any water until they start solid foods.
For formula-fed babies under 6 months in hot weather, the NHS notes that small amounts of cooled, previously boiled water can be offered between feeds. This is a more conservative UK guideline. In the U.S., the AAP generally recommends sticking to formula alone and offering more frequent feeds instead. If you’re concerned about dehydration (fewer wet diapers, dark urine, dry mouth, no tears when crying), contact your pediatrician rather than offering water on your own.
When Babies Can Start Drinking Water
Around 6 months, when solid foods are introduced, small sips of water become both safe and helpful. The AAP recommends 4 to 8 ounces per day (about half a cup to one cup) for babies between 6 and 12 months. This is meant to complement meals, not replace breast milk or formula, which should remain the primary source of nutrition through the first year.
Offer water in an open cup, sippy cup, or straw cup rather than a bottle. This helps develop drinking skills and makes it easier to control how much the baby takes in. There’s no need to force it. Some babies take to water quickly, others barely sip. Either is fine at this stage, since breast milk or formula still provides the bulk of their hydration.
If Your Baby Accidentally Drank Water
If your baby took a few sips of water from a cup or bottle by accident, don’t panic. A small, one-time amount is unlikely to cause harm in an otherwise healthy infant. The danger comes from repeated or large-volume water intake, such as consistently diluted formula, water given as a regular drink between feeds, or large bottles of water offered to soothe a fussy baby.
After the accidental sip, simply return to normal breast milk or formula feedings. If your baby was given a significant amount of water (several ounces) and begins showing signs like unusual sleepiness, puffiness, vomiting, or irritability, seek medical attention promptly. Quick correction of sodium levels typically leads to a full recovery.