Actually a 5 month old should not be having water. Babies don't need water until 1 year of age. Here is some parts of an article on it.
Pediatricians at Johns Hopkins Children’s Medical Center sounded the alarm recently that water isn’t a healthful beverage to give babies under one year of age. Every year, they see healthy children being rushed to their emergency room suffering seizures after having been given water to drink. It takes very little water to lead to water intoxication in babies and tiny tots. As little as three ounces a day could be too much water for a child under a year old, said Dr. Allen J.Walker, M.D., head of the Emergency Department at Hopkins Children’.
As their notice said:
Too much water dilutes sodium in the blood and flushes it out of the body, thus altering brain activity, which can lead to a seizure. Infants under 1 year of age may be more prone to these types of seizures than older children because a young infant’s diet does not contain enough food sources to replenish the lost sodium. Also, an infant’s immature kidneys cannot flush out excess water fast enough, causing a dangerous buildup of water in the body.
Breast milk and formula are the best way to keep a child under 1 year of age who is not eating solid foods hydrated, pediatrician Allen Walker says, and straight water should be avoided. Over-diluted formula can lead to water intoxication as well. Electrolyte-enriched pediatric drinks are not recommended for routine hydration.
Symptoms of water intoxication in an infant include:
� changes in mental status, i.e., unusual irritability or drowsiness
� low body temperature, usually 97 degrees or less
� facial swelling or puffiness
� seizures
While these seizures generally do not lead to permanent health problems, they are preventable. A parent’s natural instinct in hot weather is to give their infant water to prevent dehydration, said Dr. Walker, but if they need extra fluid, breast milk or formula is best.