While sippy cups aren’t inherently bad, a prolonged use can cause malocclusion (crooked teeth), dental decay, and muscle imbalances (1).
Our tongue posture growing up directly affects the outcome of our teeth later on in life.
This is a great article written by myofunctional therapist, Jill Savolt. I highly encourage you to read the full article, but see the highlights below.
A sippy cup is meant to be used SHORT-TERM as a child transitions from the breast/bottle to an open cup. Children should begin transitioning away from a sippy cup and to an open cup at around 6 months, as this is the time they start to develop the proper muscle functions of the tongue, lips and throat, and develop their mature swallow.
The reason why you do not want to prolong the use of sippy cups is because the spout teaches incorrect tongue placement.
When you swallow, your tongue should RISE to the roof of your mouth and behind your teeth. When you drink from a sippy cup your tongue is forced to DROP underneath the spout and into the floor of the mouth, encouraging all the wrong muscles!
Open cup drinking helps to promote proper tongue placement and oral motor skills. According to Diane Bahr, author of Nobody Ever Told Me (or my Mother) That!: Everything from Bottles and Breathing to Healthy Speech Development (2) open cup drinking should begin around 5-6 months of age. This is when your child’s lip and tongue movements are becoming more independent from their jaw movements and jaw stabilization begins.