Coffee is acidic, says Heather Kunen, DDS, an orthodontist at Beam Street in New York City. (The same is true of other acidic beverages, like teas.) When you drink it, the pH levels in your mouth drop, a change that softens your enamel and makes it more vulnerable to erosion.
If you brush your teeth too soon after drinking coffee, you risk stripping away some of that protective layer over time. "Enamel is not a continuously growing tissue, so once it's removed, it cannot be restored," Kunen says.
The bottom line:
Immediately post-coffee, rinse your mouth with water to jumpstart the pH reset and remove stain-causing residue, Kunen says. Wait at least 20 minutes to brush your teeth.