The dry mouth sensation
One other reason that caffeinated drinks are commonly associated with dehydration, according to Webster, is because some people experience a dry mouth feeling after drinking them.
“This is due to tannins, which are compounds found in tea, coffee, some fruits, and even dark chocolate,” she said. “When we eat or drink these foods, tannins bind to our saliva, causing that drying, astringent feeling.”
However, she said this doesn’t translate into dehydration, since these drinks are mostly water.