Honestly, most patients ask this before they even book, and so what's the honest answer? Almost nothing happens to the tooth itself, but a small amount has to go.
Imagine it like this. Honestly, the veneer itself is about 0.3 to 0.5 millimeters thick. About the thickness of a fingernail. Honestly (your dentist needs to remove that exact amount of enamel from the front surface)so the veneer sits flush and doesn't look bulky. So the prep work? Not a deep shave at all. In most cases (it's just a bit more than a polish)barely.
Patients often walk in thinking their teeth are getting filed down to a nub. You're not getting a crown here. In reality (reduction happens almost entirely on the front face)the labial surface, in dental terms. The back, the biting edge, the sides, those are left largely alone. Your tooth stays alive (nerve intact)because the enamel gets thinned, not removed.
Most guides skip this bit: enamel removed during prep is gone for good. Enamel doesn't grow back. So once veneers go on (you're in for the long haul)either them or replacements later.
Honestly, which is why this decision matters.
But the real interrogative: what happens to the tooth underneath, and does it rot? And can it get cavities? Truth is, sure, but the veneer itself isn't to blame. Honestly, actually, the edges are the risk zone. In practice, plaque builds up along the gumline when that margin isn't sealed perfectly, or if you neglect cleaning. Decay begins there, not underneath the porcelain.
Honestly, that's the main reason I tell people going to Turkey for E-max veneers to be picky about the lab and the dentist. When a veneer doesn't fit right, it leaves a gap. Food gets trapped in that gap, and that's where the real trouble starts. Trapped food leads to decay under the veneer. In practice, under a veneer, decay is a nightmare, you usually have to remove the whole thing to fix it.