Sensitivity is the single biggest reason people stop whitening before they finish. The kit gets put in a drawer on day four, half used, and the result never arrives.
Most of it is preventable. Here's what's actually happening and what the evidence says helps.
What causes it
Peroxide is a small molecule. It diffuses through enamel and dentine to reach the pigment molecules causing discolouration — that's the whole point, and it's why whitening works at all.
On the way through it can reach the dentinal tubules, the microscopic channels running toward the nerve. Fluid movement in those tubules is what triggers the sharp, short response you feel — typically to cold.
There's a second cause that gets misdiagnosed constantly: gel on the gums. If your tray doesn't seal, gel escapes onto soft tissue. That produces a dull, lingering ache along the gumline, which feels like tooth sensitivity but isn't. It's a fit problem, not a formula problem.
What the evidence says helps
Potassium nitrate
This is the best-evidenced option for whitening sensitivity specifically, and it isn't close.
A systematic review and meta-analysis published in the Journal of Dentistry in 2015 pooled trials on desensitising agents used during bleaching. For the proportion of patients who experienced sensitivity at all, the pooled odds ratio was 0.45 (95% CI 0.28–0.73). For how intense that sensitivity felt, the pooled standardised mean difference was −0.47 (95% CI −0.77 to −0.18).
Both statistically significant, and importantly, no negative effect on the whitening result itself.
Potassium nitrate works by raising potassium ion concentration around the nerve, which reduces its ability to fire. It doesn't block the tubules — it makes the nerve less reactive.
Nano-hydroxyapatite
Hydroxyapatite is the mineral enamel is largely made of, so a synthetic nano-scale version is chemically at home on a tooth surface. It works differently to potassium nitrate: rather than calming the nerve, it deposits mineral and occludes tubules.
A 2015 randomised trial in the International Journal of Dental Hygiene tested a bleaching gel containing nano-hydroxyapatite against the same gel without it, in 60 participants. The nano-hydroxyapatite group had significantly lower sensitivity (p<0.05), with equivalent whitening.
Worth being straight about the limits: a 2019 meta-analysis of nano-hydroxyapatite in dentin hypersensitivity found clear benefits for evaporative and tactile triggers, but no significant benefit for cold specifically (SMD −0.17, 95% CI −0.81 to 0.48). Since whitening sensitivity is largely cold-triggered, that matters. It's a useful supporting ingredient, not the headline.
Lower concentration, not shorter wear
This one is counterintuitive and worth knowing.
A 2025 randomised trial in the Journal of the American Dental Association compared 2, 4 and 8 hours of daily wear at the same gel concentration over 14 days. Sensitivity risk and intensity were statistically similar across all three durations. Wearing trays longer did not make people more sensitive.
What did predict sensitivity was concentration. A 2018 meta-analysis in Operative Dentistry found 10% carbamide peroxide carried significantly lower sensitivity risk than more concentrated gels (OR 0.41, 95% CI 0.20–0.84), with no significant difference in whitening efficacy.
So if you're sensitive, the lever is strength and frequency — not shaving minutes off each session.
What you can do tonight
- Use less gel. About a grain of rice per tooth. If it squeezes over your gums when you seat the tray, that's too much, and that's the part that stings.
- Check your tray fit. You should see the outline of individual teeth in a moulded tray. If it looks like a smooth U, it hasn't taken — remould it.
- Go every other day. Result depends on total contact time, not consecutive days. You'll arrive a few days later feeling considerably better.
- Take a day off if you need one. A missed day costs almost nothing. Pushing through real pain costs you the rest of the course.
- Brush with a hydroxyapatite or potassium-based toothpaste between treatments.
Normal versus not normal
Normal: mild sensitivity to cold, appearing during or shortly after a treatment, settling within hours. In one randomised trial, sensitivity returned to baseline within seven days of finishing, with nobody reporting it at the final check.
Worth stopping for: sharp pain focused on one specific tooth — that can indicate a crack, an existing cavity or a failing restoration that whitening has found. Also gum soreness still present the next morning, which usually means gel is escaping the tray.
Neither means you can't whiten. Both mean something needs adjusting before you continue, and the second one is nearly always tray fit.
One caveat on the numbers: a 2015 systematic review concluded that published estimates of how common whitening sensitivity is vary too widely, with too much risk of bias, to state a reliable figure. Anyone quoting you a precise percentage is quoting something that isn't well established.
References
Wang Y, Gao J, Jiang T, Liang S, Zhou Y, Matis BA. Evaluation of the efficacy of potassium nitrate and sodium fluoride as desensitizing agents during tooth bleaching treatment — a systematic review and meta-analysis. Journal of Dentistry. 2015;43(8):913–923.
Vano M, Derchi G, Barone A, Genovesi A, Covani U. Tooth bleaching with hydrogen peroxide and nano-hydroxyapatite: a 9-month follow-up randomized clinical trial. International Journal of Dental Hygiene. 2015;13(4):301–307.
de Melo Alencar C, de Paula BLF, Guanipa Ortiz MI, Baraúna Magno M, Martins Silva C, Cople Maia L. Clinical efficacy of nano-hydroxyapatite in dentin hypersensitivity: a systematic review and meta-analysis. Journal of Dentistry. 2019;82:11–21.
de Geus JL, Wambier LM, Boing TF, Loguercio AD, Reis A. At-home bleaching with 10% vs more concentrated carbamide peroxide gels: a systematic review and meta-analysis. Operative Dentistry. 2018;43(4):E210–E222.
Kielbassa AM, Maier M, Gieren AK, Eliav E. Tooth sensitivity during and after vital tooth bleaching: a systematic review on an unsolved problem. Quintessence International. 2015;46(10):881–897.
Our full claims and the research behind them are on our Claims & Evidence page.