Whitening ads sell a color change without explaining what part of the tooth is actually changing. A tooth is not one solid material. The outer shell is enamel, which is hard, mostly mineral, and semi translucent. Under it sits dentin, which is softer, more yellow, and makes up most of the tooth. What you read as tooth color is mostly dentin showing through enamel, plus whatever has stained the surface. Peroxide breaks apart the pigment molecules sitting in and on those layers. It does not add white. It removes color.
Stain comes in two forms and they do not respond the same way. Extrinsic stain sits on the outside, in a thin protein film called the pellicle that coats teeth within minutes of brushing. Coffee, black tea, red wine, dark sodas, curry, and tobacco all bind to that film. Intrinsic stain lives inside the dentin and comes from aging, from a tooth that took a hit years ago, from tetracycline exposure in childhood, or from too much fluoride while teeth were still forming. Whitening handles extrinsic stain well because the gel only has to reach the surface. Intrinsic stain takes longer, needs more contact time, and sometimes never fully lifts.
The active ingredient is the same across nearly every product on the shelf. It is either hydrogen peroxide or carbamide peroxide, and carbamide peroxide breaks down into hydrogen peroxide at roughly one third the strength. A ten percent carbamide gel is close to a three and a half percent hydrogen peroxide gel. Store strips usually run between five and fourteen percent hydrogen peroxide. Take home trays from a dental office typically use ten to twenty two percent carbamide. In office treatments go much higher, often twenty five to forty percent, with gum barriers placed first. Price gaps between brands are mostly packaging, fit, and supervision rather than chemistry.
Here is the part that catches people. Peroxide only whitens natural tooth structure. Crowns, veneers, bridges, and tooth colored fillings are ceramic or resin, and they do not react to the gel at all. If you have a composite filling on a front tooth, or a crown on a tooth that had a root canal, those pieces stay the exact shade they were made. Whiten everything around them and the restoration that used to blend now reads dark by comparison. Anyone with bonding on a chipped front tooth learns this the expensive way. The only fix is replacement.
Sensitivity is the most common side effect and the mechanism is worth knowing. Peroxide is a small molecule and it travels through enamel into the dentinal tubules, which are microscopic channels running toward the nerve. That traffic irritates the pulp and produces short, sharp zings with cold air or cold drinks. It is usually temporary and settles within a few days of stopping. Gels with potassium nitrate and fluoride reduce it, which is why those ingredients appear on so many labels. Gum irritation is a separate problem and usually means the tray does not fit or you overfilled it. A session should not leave you in real pain.
Charcoal pastes and most whitening toothpastes are doing something else entirely. They are abrasives, and they lift surface stain by scrubbing rather than by bleaching. Abrasion is measured on a scale called relative dentin abrasivity, and higher numbers mean more wear on the tooth. Enamel does not grow back, so anything that thins it permanently makes teeth look more yellow over time, because more dentin shows through. The American Dental Association has not awarded its Seal of Acceptance to charcoal dentifrices and has pointed to thin evidence on both safety and effect. A polishing paste can make teeth look cleaner. That is not the same as changing their shade.
Results do not hold forever and the fade pattern is predictable. Teeth rebound slightly in the first week or two as they rehydrate, so the shade you see on day one is not the shade you keep. After that, most people hold a result somewhere between six months and two years, depending on what they drink and whether they smoke. Touch up trays extend it far more cheaply than starting over with a full treatment. Drinking dark liquids through a straw and rinsing with water afterward does more for upkeep than any product sold for it. The habit matters more than the purchase.
If you are going to do it, the order of operations saves the most money. Get a cleaning first, since removing buildup alone changes how light hits the tooth. Ask which of your front teeth carry restorations and get that answer before you buy anything. Whiten to the shade you want, wait about two weeks for the color to settle, and only then have crowns or fillings matched to the new shade. Doing it in the other order guarantees a mismatch you will pay to correct. Expect under a hundred dollars for strips, a few hundred for custom trays, and several hundred or more for in office work.
