After a few days of using whitening strips, your teeth will become more sensitive and that is normal because it is a well known side effect. In case you're worried, the sensitivity should self-resolve once you stop the whitening treatment.
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Whitening strips can make your teeth sensitive
According to the American Dental Association (ADA), teeth sensitivity is one of the two main side effects from whitening. Therefore you didn't do something wrong because your teeth are feeling sensitive from the treatment.
It is within normal limits for them to be feeling some sensitivity but they shouldn't be "killing you". Studies have found that there is a correlation between the concentration of the product and the amount of sensitivity and irritation experienced.
The primary reason why they feel sensitive is due to the hydrogen peroxide within the whitening strips. It is the peroxide which bleaches the teeth but they're also the cause for the sensitivity. Therefore, it's not necessarily "bad" that they cause discomfort, it's a normal part of the process.
Why teeth become sensitive from whitening strips:
Bleaching gel is potent enough to reach the tooth nerve.
Dislodges and dissolves smear plugs.
Hypertonic solution via hydrodynamic theory
Peroxide diffuses to the pulp
The whitening strips are indeed applied to the exterior surface of the enamel. You may think that the bleaching gel only affects the outer layer of your tooth but that couldn't be further from the truth.
The peroxide within the strips are potent enough to diffuse through every layer of your tooth and can even reach the pulp. Yes, studies have shown that after using peroxide based whitening products for 15 minutes, it can be found at tooth nerve.
As unpleasant as that may sound it is also the reason why hydrogen peroxide is able to bleach your teeth the way that they do. If it wasn't able to penetrate the tooth, it wouldn't be able to remove the deeply embedded intrinsic stains.
If you think about it, peroxide is an acid and the nerve is very sensitive to stimuli. Now you're placing the acid next to a very sensitive tissue, that could be a recipe for a lot of sensitivity!
Dislodges and dissolves the smear plugs
Our body has a natural defense mechanism that prevents us from feeling sensitivity to various external stimuli. That protection comes in the form of a smear plug, which blocks off the openings of all the dentinal tubules which lead to the tooth nerve. It serves as a physical barrier between the nerve endings and the stimuli.
How the peroxide in the whitening strips make your teeth sensitive is in their ability to dislodge and dissolve these smear plugs. Essentially all of these plugs get removed which effectively opens up all of the dentinal tubules for stimuli.
Put another way, there is absolutely nothing stopping or impeding the acidic bleaching gel from waltzing right into the tubules. It is like an open buffet for the peroxide to feast upon the nerve endings.
Peroxide is very hypertonic
According to Branstorm's hydrodynamic theory for dentinal hypersensitivity, the hypertonicity of the peroxide causes fluid movements in the dentinal tubules. Receptors can sense these fluid movements which results in a sensitivity signal being generated.
Since the peroxide is very acidic and concentrated, it would pull fluids away from the nerve. That change in fluid movement is thought to be the major influencer on sensitivity. This goes hand in hand with the removal of the smear plugs because it potentiates the effect.
Therefore the concentration of the peroxide would have a large effect on tubular fluid movement. The more concentrated the greater the pull. Typically the whitening strips are more concentrated than toothpastes.
Although the same could be said about in-office bleaching sessions which are significantly more potent than at home products! That means you should expect more sensitivity if you whiten your teeth at the dentist.
How long does the sensitivity last after whitening strips?
According to the ADA, the sensitivity from using strips to whiten your teeth should only be mild and transient. Studies have shown that sensitivity may afflict up to two thirds of patients undergoing treatment.
It'll typically start feeling sensitive after the second or third day of treatment for whitening strips and at home tray treatments. The good news is that the sensitivity should resolve approximately four days after finishing the entire treatment.
In other words, it starts being sensitive after about the second day and will continue to be sensitive as long as you keep using it. Although it should cease about four days after you stop using the strips.
Tip: If you want to decrease the sensitivity during whitening, you should've started using a desensitizing toothpaste like Sensodyne Pronamel two weeks before starting. That usually helps to reduce the symptoms during treatment. You should look for toothpastes that have the ingredient potassium nitrate along with sodium fluoride.
How do you get rid of sensitive teeth after whitening strips?
Depending on how severe the sensitivity is from using the whitening strips, you need to make a decision about whether you want to continue or not. Since the adverse effects are only temporary in nature, the sooner you stop using the product the faster it'll go away.
However if you want the maximum whitening effect you would need to finish the treatment as prescribed. Nonetheless, it doesn't make sense to suffer through it if it is too severe.
Tips on how to reduce teeth whitening sensitivity:
Take painkillers. They work for more than just fevers and back pain because they can also reduce teeth sensitivity.
Use a desensitizing toothpaste. Look for one with ingredients such as potassium nitrate, stannous fluoride, or nano-hydroxyapatite. They either inactivate the nerve or recreate smear plugs to occlude the open dentinal tubules.
Ask your dentist for fluoride treatment. Studies have shown that fluoride varnish can reduce hypersensitivity. The perk is that the fluoride does not interfere with the bleaching process.
Prescription desensitizer. Your dentist may have a special desensitizer that you're unable to buy OTC. It can be either calcium oxalate or GLUMA.
As an additional tip, you should wait 30 minutes before brushing your teeth with a desensitizing toothpaste after whitening. The reason is because you want the mouth to neutralize the residual acid on your enamel before you go brushing them. You don't want to rub acid directly into the softened enamel from the bleaching gel.
It is common to experience some sensitivity while using whitening strips since they do contain peroxide. The strips also come in a higher concentration of hydrogen peroxide than regular whitening toothpaste. That adds to the sensitivity discrepancy when you compare the two.
Fortunately for you it is safe to whiten with the strips despite the potential side effect. The adverse effect goes away all on its own without you doing anything if you give it enough time. Give it about 4-5 days after using the product for everything to go back to normal.