Hot water with lemon has become a morning fixture for a lot of people. It is also one of the more common causes of the enamel wear we see on the inside of upper front teeth in otherwise healthy mouths. Both of those things can be true at once.
The problem is acidity, not sugar
Lemon juice sits at roughly pH 2 to 2.5. Enamel begins to demineralise below about pH 5.5, so even heavily diluted lemon water is comfortably in the range that softens it. This is erosion rather than decay: no bacteria involved, just acid dissolving mineral directly off the tooth surface.
Saliva does repair this, but it takes time. It is the frequency of acid exposure that does the damage, far more than the amount. Sipping one mug slowly over an hour is considerably worse than drinking the same mug in five minutes.
What erosion looks like before it is obvious
Early signs are easy to miss. Front teeth start to look slightly translucent at the edges, almost grey where the enamel has thinned and the darker dentine shows through. Teeth can look shorter. Cold drinks start to catch. Older fillings begin to stand slightly proud of the tooth surface, because the filling material has not worn but the enamel around it has.
By the time the change is obvious in the mirror, a fair amount of enamel has already gone, and unlike a cavity it cannot be cleaned out and filled. It is replaced, if at all, by adding material on top.
How to keep drinking it
Drink it in one go rather than sipping. Use a straw if you can, so it bypasses the front teeth. Rinse your mouth with plain water afterwards. Follow it with something alkaline, such as milk or a piece of cheese, which neutralises the acid faster than saliva alone.
And wait before brushing. Softened enamel is vulnerable, and brushing straight after an acid drink physically removes some of it. Give it an hour, or brush before your lemon water rather than after.
The rest of the list
Lemon water is rarely the only culprit. Sparkling water is mildly acidic. So are fruit teas, most fizzy drinks including the sugar-free ones, wine, and cider vinegar taken neat as a supplement, which is the worst of the lot. If several of those are in your daily routine, the effect stacks.
If the damage is already there
Mild erosion is usually managed rather than treated: high-fluoride toothpaste, habit changes, and photographs at each check-up so we can see whether it is progressing or stable. Where teeth have visibly shortened or become sensitive, composite bonding rebuilds the worn edges in a single visit without drilling into what enamel remains.