How Does Hydrogen Peroxide Whiten Teeth?

Written & Reviewed by Dr David Chen

There is no doubt that hydrogen peroxide (HP) can whiten teeth because if it didn’t, the industry wouldn’t be as large as it is today. We’ve literally lost count of how many whitening products there are on the market.

From strips, pens, mouthwashes, toothpastes, customized trays, and in-office whitening sessions. These all contain the miraculous whitening ingredient of hydrogen peroxide or at least some derivative of it.

hydrogen peroxide - floor

As evidence that it works, this study by the Journal of Dentistry found that a product containing just 6% hydrogen peroxide not only removed extrinsic stain but also increased the tooth whiteness. These results were statistically significant when compared to a placebo gel with no HP.

In case you were not convinced, you can also ask any of your friends who have done teeth whitening. They’ll tell you all about how well it worked! This study found that over a 2 year follow up period after whitening, there was an objective color difference. Subjectively, there was no reported difference though. Isn’t that fascinating?

However you may be curious as to how hydrogen peroxide whitens your teeth because you can literally buy a bottle of it at the local pharmacy. Can it really change the shade of your teeth from yellow to bright white? What is the mechanism that it uses to do this? Is it even safe to get your teeth white like this or are there side effects?

How hydrogen peroxide whitens teeth

Hydrogen peroxide whitens teeth by chemically oxidizing stains as it diffuses through the enamel, dentin, and pulp. It is able to do this because it is an unstable substance that readily decomposes in the presence of catalysts, heat, and light. Peroxide ultimately breaks down into water, oxygen, and heat.

hydrogen peroxide decomposition one step

However, the steps in between create powerful free radicals that can oxidize extrinsic and intrinsic stains in teeth. The radicals interact with more peroxide and create more of itself, causing a self-propagating chain reaction.

hydrogen peroxide decomposition to radicals - reaction equations

These free radicals are very potent in that it affects both types of stains. Yes, that means it can penetrate through the entire tooth and even end up at the pulp.

Therefore it can oxidize intrinsic stains that are deeply embedded within the tooth which is hard to get to. However, it appears that hydrogen peroxide only oxidizes the organic matrix of the tooth while leaving the inorganic portion intact.

Significance of oxidizing the organic matrix

Since hydrogen peroxide only oxidizes the organic portion of the tooth, it means the dentin is a juicy oxidizing target. The reason is because the dentin layer contains significantly more organic matrices than the enamel.

The enamel layer is about 98-99% inorganic by weight which leaves roughly 1-2% of it being organic. The dentin has about 10x as much organic substances by weight in comparison.

This may be eye opening for you since most people believe that the whitening action occurs only at the enamel while in fact, it is actually the dentin that has the potential for most changes in color. Surprisingly, the enamel doesn’t have that much to whiten since it is mostly inorganic.

Why does oxidation make teeth whiter?

Tooth stains or chromogens consist of many double conjugated bonds. The more of these bonds each stain has, the more light it can absorb and the less of it it will reflect. In other words stains with more double bonds means it will appear darker in color.

Hydrogen peroxide oxidation of double bonds
Credit: Clifton Carey

What hydrogen peroxide does or rather the free radical does is convert the double conjugate bond to a single bond. What effectively happens is a decrease in the number of double conjugated bonds in the stain. Therefore light absorption is decreased and light reflection is increased.

Basically the stain becomes “invisible” to our eyes. Technically the stain is still there but we just can’t see it because it is now reflecting all of the light so it appears white.

Almost sounds like a conspiracy doesn’t it?! It makes sense if you think about how black light can reveal stains. Ain’t that interesting?

Related content: How does teeth whitening work.

Tooth anatomy and their organic to inorganic ratios

The crown of a human tooth is composed of three distinct layers that contain a mix of inorganic to organic substances. Each layer has a different ratio and that gives them their distinct texture and hardness.

  • Enamel. This study demonstrated that it consisted of mostly an inorganic matrix with only 1% of it being organic by weight.
  • Dentin. This study found that an organic matrix consisted of 20% of the entire structure by weight with the rest being inorganic.
  • Pulp. The nerve chamber of the tooth is mainly composed of organic substances, which is why it is soft and not hard.
Human tooth diagram - KDS4444
Credit: KDS4444


Since the enamel contains mostly inorganic substances (98-99%), it also makes it the hardest layer of the tooth. To be precise, it is actually the hardest substance in the human body. It even exceeds your bones in hardness. The enamel has a mohs hardness scale of about 5 which is harder than copper and your fingernail.

That is the reason why it is the outermost layer so that it can serve as protection against all of the elements. It will protect your teeth while you’re chewing and also from varying temperatures of foods that you may be eating.


The dentin layer has a larger ratio of organic composition (20%), which permits it to be oxidized by hydrogen peroxide. You may have initially thought that it was the enamel being whitened but it turned out that it was the dentin instead.

As surprising as it may have been, it does make sense because a side effect of bleaching your teeth is teeth sensitivity. Since the enamel is mostly inorganic, it doesn’t have any nerve endings in there so it doesn’t feel anything. The dentin on the other hand has a LOT of nerve endings embedded within it so if you’re feeling sensitivity, it is definitely because the whitening gel has reached it.


The pulp is mostly organic with very little inorganic substances within it. The reason is because this is the softest layer out of the entire tooth since it houses the majority of the nerves and blood supply.

This layer doesn’t really contribute to the color of the tooth since it is the innermost layer. The color of the enamel and dentin blended together make up the shade of your tooth which you can see.

What is also important to note is that hydrogen peroxide can penetrate as deep into the tooth as the pulpal layer. Studies have shown that by the 7th hour of whitening, HP will have diffused their way into the pulp chamber.

You may not care about your pulp being bleached white since it does not contribute much to the overall tooth shade but you will care that it houses the bulk of your tooth nerve. What that means is whenever you feel severe teeth sensitivity from whitening, it is because your pulp has come into contact with hydrogen peroxide!

Is it safe to whiten your teeth with hydrogen peroxide?

Whether teeth whitening is safe or not is relative to the concentration of hydrogen peroxide in the product that you’re using. In other words, the lower the concentration of HP the safer it is for you. The vice versa is also true where the higher the concentration, the higher the risk for adverse side effects.

Side effects:

  • Burned gums. The hydrogen peroxide is very potent because it will not only bleach your tooth but also the gums. If that happens you’ll notice the gums turn white but the good news is that you can heal the burned gums with home remedies.
  • Teeth sensitivity. A major side effect of whitening is sensitivity. It’ll be sensitive for a few days even after stopping treatment. You’ll feel it when you suck in cold air and drink cold beverages.

Alternatively there are also peroxide free teeth whitening products on the market. You can give those a try if you’re concerned about using too much hydrogen peroxide.

Research with no adverse effects

According to a study in the Journal of Esthetic and Restorative Dentistry, no adverse effects were found when using low concentrations of hydrogen peroxide.

  • Enamel morphology remained intact.
  • Microhardness of enamel was unaffected.
  • Pulpal enzymes were not inhibited.

That is good news according to this study but you must be aware that the study tested hydrogen peroxide at varying levels of low concentration. They examined products with a HP concentration of 3.5%, 7%, and 12% which comes in at the lower end. There are plenty of professional in-office products where the concentration level of HP exceeds 30%.

Opalesence Go

Nonetheless, what this study implies is that the vast majority of OTC whitening products are most likely safe for you to use without direct supervision by a dentist. Consequently, it is also why the higher concentration products are reserved for in-office use under direct supervision of a dental professional.

Research with adverse effects

A study by the Journal of Dentistry, examined the toxicity of tooth bleaching protocols on the pulp. They tested varying concentrations of hydrogen peroxide as well as the amount of time the tooth was put under whitening.

Kor Whitening take home kit

There was only one instance of toxicity and that was from 35% HP used for 3 rounds of 15 minutes. All the other groups and variations exhibited cytotoxicity to the pulp but eventually recovered.

Here were the experimental groups that were tested:

  • 35% – 3 x 15 mins
  • 35% – 1 x 15 mins
  • 35% – 1 x 5 mins
  • 17.5% – 3 x 15 mins
  • 17.5% – 1 x 15 mins
  • 17.5% – 1 x 5 mins

Their overall recommendation was to simply reduce the 35% concentration and you would achieve safety. However, they also stated that if you reduced the contact time of the 35% by 5 minutes you would also achieve the same safety result.

To summarize, a higher concentration of HP can still be used safely if you reduce the amount of treatment time. A lower concentration of HP can be used safely for a much longer period of treatment time.

Last but not least, an often overlooked condition is bleachorexia which can result in adverse effects for the mouth. This is actually not done by highly concentrated whitening gels but rather due to a long term abuse of low concentrated OTC products. It is simply because they’re easily accessible for purchase online.

Factors that influence whitening efficacy

In vivo conditions have various other factors which can influence the whitening effectiveness of hydrogen peroxide. There are factors which make it more effective and also less effective.


Whitening gels that are stored in a fridge under cool conditions will have a longer shelf-life. They stay effective longer and won’t go bad as quickly.

If you leave your bleaching gel out, it can potentially go bad and lose its effectiveness. This is one of the main reasons as to why some OTC whitening products don’t give great results. It’s because they’re not refrigerated.


It is a myth that low concentrations of hydrogen peroxide can’t whiten to the level of a higher concentration product. This study by the British Dental Journal found that all whitening approaches will whiten teeth because bleach is bleach so the concentration is irrelevant.

The study tested products which contained 3% hydrogen peroxide and all the way up to 35% products. They all achieved the same cosmetic results but at different lengths of treatment time.

Their conclusion was that the concentration of hydrogen peroxide affects the time it takes to reach the desired level of tooth whiteness. In other words, the more HP there is in the product, the less time it will take to whiten your teeth. Conversely, the lower the concentration the longer you have to whiten with it to achieve the same result.

Therefore, what influences the efficacy of teeth whitening depends on the concentration of the product and the total exposure time. If you decrease one variable, you will have to increase the other one to achieve the same result.

One of the reasons why people often think that their hydrogen peroxide is not whitening their teeth is because they’re using a low strength product. However that couldn’t be farther from the truth because they simply haven’t used it for a long enough time. The reason why they aren’t seeing immediate results is due to insufficient usage time.

pH of whitening gel

Hydrogen peroxide is acidic in nature but it actually works more effectively in an alkaline environment and not an acidic one. Studies have shown that as you steadily increased the pH of the gel, it becomes more potent with whitening up to a pH of 9.

effect of ph on peroxide whitening efficacy
Whitening efficacy of peroxide in different pH – Credit: CRG Torres; E Crastechini; FA Feitosa; CR Pucci; AB Borges

The chart above shows how with each incremental increase in the pH, more wine and tobacco stains were removed. The conclusion was that hydrogen peroxide is more effective in an alkaline solution!

However that is also a double edged sword.

  • Peroxide is more reactive and will decompose faster in alkaline solutions but that makes it have a poor shelf life.
  • Storing peroxide in an acidic solution will extend its shelf-life but it won’t be as effective whitening wise.


Saliva is the arch nemesis of all whitening agents, especially hydrogen peroxide. Our bodies naturally have salivary peroxidases which decompose it in order to protect ourselves from toxification.

It is not good for our whitening progress when saliva starts pooling and coming into contact with the gel. It starts to break it down and render it ineffective. That is why whitening at the dentist is advantageous because they have a saliva suction that helps to remove some of it.

Related content: how to stop saliva while whitening.

How long does the whitening last?

According to a study done by the Journal of Esthetic and Restorative Dentistry, the whitening effect can be effectively maintained for up to six months. This was done with a low concentration product which only contained 3% hydrogen peroxide. It was used for 30 minutes three times a day for 2 weeks total.

The study did not go on further than the six month mark but since the color held until that point, we can safely assume that it probably lasted longer than that. Most likely a single whitening treatment should theoretically last you longer than six months.

Of course, how long it lasts will have a lot to do with your dietary habits such as if you consume a lot of staining foods and drinks. The less of those you consume, the longer the color probably maintains. Overall, it is probably a very individualized result.


Hydrogen peroxide can whiten your teeth by diffusing its way through the entire tooth and oxidizing organic substances along the way. However it leaves the inorganic portions untouched which means that it mostly whitens the dentin and not the enamel. That may be different than what you may have originally thought.

Despite HP being able to penetrate all the way to the pulp, it is still relatively safe but it depends on the concentration and exposure time. The pulp can recover from being in contact with hydrogen peroxide except when it is a high concentration for an extended period of time. The lower concentration products don’t seem to cause any irreversible damages so OTC products are safe.

Last but not least, what is interesting to note is that all concentrations of teeth whitening products can achieve the same result. It is just that a high level of it will achieve it in a shorter period of time. A lower level product simply requires a longer exposure time to do so. In other words, professional teeth whitening will give you faster results and you basically pay to have same day delivery!

Well in a nut shell, thats what HP can do for your teeth. Are you curious as to what peroxide-free whitening can do? Heh.


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