I give Lexie it every other day. She only drinks about 1oz.
She hasn't poo'ed in 2 days, today is the 3rd day, so I've give her some today to try & soften her poo, cause she must be constipated. I was told only to give her water if her poo look's hard. But babies do get thirsty to, just like us. So every other day. It's doing her no harm.
Here we aren't suppose to give water until they are 6 months old. Before then their bodies cannot process plain water, and can die from water toxicity. They can also choke on plain water. We didn't start giving Hunter water until he was about 9 months old. Before then he just didn't need it, he stayed hyderated with his formula. At 9 months we started giving him a little in his sippy cup with his meals.