Soothely
The tubules, the nerve, and the reason the sting always shows up about an hour later.
There is a specific, well-documented reason your teeth ache the evening after a whitening treatment, and it has nothing to do with the strips being faulty or you leaving them on too long. It is how peroxide works, and no amount of careful application changes it.
Enamel is not a solid shell. It is a dense crystalline layer, and under it sits dentine, which is threaded with somewhere between fifteen and fifty thousand microscopic channels per square millimetre. These are the dentinal tubules, and they run from just under the enamel down to the pulp, where the nerve lives.
In a healthy tooth those channels are plugged with mineral and a small amount of fluid. That plug is what stops a mouthful of cold water registering as pain. Anything that removes it exposes a direct hydraulic path to the nerve.
Hydrogen peroxide, or carbamide peroxide which breaks down into hydrogen peroxide in your mouth, is a very small molecule. Small enough to pass straight through intact enamel and into the dentine underneath. Once there it releases free radicals that attack the long pigment molecules responsible for staining, breaking them into shorter fragments that reflect more light. That part genuinely works, and it works quickly.
The problem is that the same oxidative reaction does not distinguish between a coffee pigment and the mineral plug in a tubule. It strips both. Every session that whitens also demineralises, and the demineralised tooth is the one that hurts.
Peroxide whitens and demineralises through the same reaction. The whitening is the point, the sting is the side effect, and they are not separable by using less or leaving it on for a shorter time.
This is the detail that convinces people the product was fine and they did something wrong. The oxidation takes time to work through enamel into dentine, and the fluid shift in a freshly opened tubule takes longer still. By the time the nerve responds you have finished, rinsed and moved on. The delay makes the cause hard to see.
It also means stopping early does not help much once the session is underway. The reaction continues after the strip comes off.
Phthalimidoperoxycaproic acid is a larger, more selective oxidiser. It attacks the same double bonds in the pigment molecules, so the stain still breaks apart, but it does not pull mineral out of the tubules on the way past. The channels stay plugged. The nerve stays insulated.
That is the entire mechanical difference, and it is why a PAP+ formula can be used twice a day by somebody who cannot tolerate a single peroxide strip. Adding potassium nitrate, which reduces nerve excitability directly, and hydroxyapatite, which is the mineral enamel is made of, pushes it further in the same direction.
It does not mean PAP+ is faster. On raw shade change over a fortnight, high-concentration peroxide is still competitive and in some trials ahead. It means you can actually finish the course. Most people who give up on whitening give up because of sensitivity, not because the product did nothing.
If you already have exposed roots, active decay or gum disease, none of this applies cleanly and you should speak to your dentist first. An open tubule caused by recession is not something a cosmetic gel is going to fix.