Whitening gels are sold on a number. 10%, 16%, 22%, 35%, 44%. The obvious assumption is that bigger means whiter, and that a 44% gel is more than four times a 10% gel.
Neither is quite right, and the reasons are worth understanding before you spend money.
Carbamide peroxide and hydrogen peroxide are different scales
The first thing that trips people up is that kits use two different active ingredients and quote both as percentages, as if they were comparable.
Carbamide peroxide is a compound that breaks down in the mouth into hydrogen peroxide and urea. It releases roughly one third of its weight as hydrogen peroxide.
So the rough conversion is:
- 10% carbamide peroxide ≈ 3.5% hydrogen peroxide
- 16% carbamide peroxide ≈ 5.5% hydrogen peroxide
- 22% carbamide peroxide ≈ 7.5% hydrogen peroxide
- 35% carbamide peroxide ≈ 12% hydrogen peroxide
- 44% carbamide peroxide ≈ 15% hydrogen peroxide
Which means a kit advertising "35% hydrogen peroxide" is far stronger than one advertising "35% carbamide peroxide." If a product doesn't tell you which, that's worth noticing.
Carbamide peroxide also releases more slowly, which is why it suits longer tray wear, while hydrogen peroxide gels tend to be used for shorter applications.
What the research says about going stronger
Here's where the intuition breaks down.
A systematic review and meta-analysis in Operative Dentistry in 2018 compared at-home bleaching with 10% carbamide peroxide against more concentrated carbamide peroxide gels, across 13 studies.
On whitening: no significant difference in efficacy.
On sensitivity: 10% carried significantly lower risk (odds ratio 0.41, 95% CI 0.20–0.84) and lower intensity. GRADE certainty was rated moderate for most outcomes.
A 2025 network meta-analysis in the Journal of Dentistry, covering 77 studies, reached a consistent conclusion: sensitivity risk tracks concentration, with higher-concentration gels carrying roughly two to three times the risk.
So the honest summary is that beyond a certain point, higher concentration buys you speed and sensitivity rather than a whiter endpoint.
Then why do stronger gels exist?
Because total peroxide exposure is what matters, and concentration is only half of that equation. The other half is time.
A stronger gel worn for a shorter session can deliver comparable exposure to a weaker gel worn for hours. For a lot of people that's a genuinely better trade — twenty or thirty minutes in the evening is something you'll actually do fourteen nights running. Eight hours of overnight tray wear often isn't.
This is the real argument for a high-concentration gel, and it comes with an obligation: the wear time has to be correspondingly short, and it has to be stated clearly. A 44% gel used on a 10% gel's schedule is just a sensitive mouth for no additional benefit.
Longer isn't a shortcut either
If you're tempted to leave trays in longer than instructed, a 2025 randomised trial in the Journal of the American Dental Association tested exactly that. 120 participants, 2 versus 4 versus 8 hours daily, same 10% gel, 14 days.
Colour change was statistically non-inferior across all three. The extra hours bought nothing.
Is stronger gel bad for enamel?
A 2017 systematic review and meta-analysis in the Australian Dental Journal pooled 13 studies on enamel microhardness after bleaching. It found no significant change with 10% carbamide peroxide over 7, 14 or 21 days.
That's reassuring, and it's worth being precise about what it covers: that research tested 10%. It hasn't been repeated across every concentration on the market. Anyone extrapolating it to a much stronger gel is going beyond the data.
What actually to look at
The percentage is one input among several, and not the most important one.
Is it carbamide or hydrogen peroxide? Otherwise you're comparing different scales.
What's the stated wear time? A strong gel with vague instructions is a bad sign.
How will the tray fit? Gel only whitens the surface it touches. This decides more of your result than the number on the box.
Is there anything for sensitivity? Potassium nitrate has a meta-analysis behind it for whitening specifically. It's the difference between finishing a course and abandoning it.
How much gel in total? Six 5ml syringes is 30ml. A kit with three syringes at a higher percentage may still be less product.
References
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.
Terra RMO, Favoreto MW, Morris T, Loguercio AD, Reis A. Effect of at-home bleaching agents and concentrations on tooth sensitivity: a systematic review and network meta-analysis. Journal of Dentistry. 2025;160:105891.
Terra RMO, Sutil E, Ferreira Cordeiro DC, Favoreto MW, Faria-E-Silva A, Best AM, Loguercio AD, Reis A. Different daily times for at-home bleaching with 10% carbamide peroxide: a randomized single-blind, noninferiority controlled trial. Journal of the American Dental Association. 2025;156(1):57–67.
Zanolla J, Marques A, da Costa DC, de Souza AS, Coutinho M. Influence of tooth bleaching on dental enamel microhardness: a systematic review and meta-analysis. Australian Dental Journal. 2017;62(3):276–282.
Our own claims, and the research behind each one, are published on our Claims & Evidence page.