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Inside Dentistry

November/December 2006, Volume 2, Issue 9
Published by AEGIS Communications


Common Questions About the Safety of Tooth Whitening

Yiming Li, DDS, MSD, PhD

Although it has been more than 16 years since vital nightguard tooth whitening was introduced to the profession, the topic of whitening safety still elicits questions, concerns, and some controversy. Thankfully, data have accumulated over the years to demonstrate that no significant long-term oral or systemic health risks have been found from tooth whitening using 10% carbamide peroxide.1-5

However, many new products are being introduced into the marketplace, such as those advertised on television. These new products and technologies are being presented to consumers with very little or no scientific research or evidence to prove their efficacy and safety. What’s more, with the advent of the Internet, much more information is available about tooth whitening, some of which is accurate and fair and some of which is based on myths.

This article will discuss three key areas related to the safety of tooth whitening. First, the definition of safety will be framed in terms of risk. Secondly, the reasons for the concern about the safety of tooth whitening will be discussed. Finally, six specific and frequently asked questions about tooth whitening will be answered.

Defining Safety

A professional understanding of safety is quite different from what patients think about safety. For consumers, something is safe if it produces absolutely no harm. Scientists, however, recognize that there is nothing that fits this definition. As a Swiss physician and chemist once said, “All substances are poisons and there are none that are not poisons. The right dose differentiates a poison and a remedy.”6

From a professional standpoint, therefore, a substance is safe if it probably will not induce toxic effects. According to the US Food and Drug Administration, a product is safe if its ingredients have a low incidence of adverse reactions or significant side effects when used according to adequate warnings and directions.7,8 This definition assumes that professionals and consumers will follow the manufacturer’s instructions for a given product.

Contributing to product safety is the fact that different people have different tolerances and needs in different situations, and these affect the risk-to-benefit relationship of a given product’s use. As a result, complete disclosure to patients of indications, contraindications, and potential effects of all possible treatment options is necessary for any dental procedure, including tooth whitening.

Continued Concern

Dental professionals know that specific types of peroxide-based tooth whitening procedures work without any serious adverse effects.1-4 There have been no major health problems reported as a result of tooth whitening. So, why does the concern over the safety of this procedure remain?

The major reason is because the active ingredient in tooth whiteners is peroxide. The most commonly used form is carbamide peroxide, which decomposes to form hydrogen peroxide and urea. Despite its use in medicine and dentistry for nearly 100 years and the fact that a person produces and metabolizes hydrogen peroxide every minute, hydrogen peroxide is still a cause for concern.9 Additionally, because tooth whiteners are a mixture of various ingredients, there is concern that possible interactions may occur because of the active nature of peroxides.9

As more became known about hydrogen peroxide, its biochemical behavior in pathology became clear. When peroxide decomposes, it forms water, oxygen, and free radicals.9,10 Schools of thought have focused on the formation of reactive oxygen species (eg, hydroxyl-free radicals) with links that have been found to many major diseases.10

Scientists are mostly in agreement that free radicals relate to our aging process and have been associated with cancers and various degenerative diseases because they can quickly react with not only proteins and lipids, but also DNA. When a mutation occurs in DNA, problems (eg, cancer, birth defects) can occur.10

The reason for safety concerns over tooth whitening lies in the question: Will the use of hydrogen peroxide for tooth whitening cause these types of diseases?11,12 Many studies have been conducted on the toxicology of carbamide peroxide and hydrogen peroxide—particularly in tooth whiteners—during recent years (eg, cytotoxicity, genetic toxicity). The answers surrounding the safety issues and lingering questions follow.

Six Frequently Asked Questions

1. Does the Peroxide in Tooth Whiteners Cause Cancer?
This question continues to be asked because (obviously) patients will not risk losing their life for the sake of whiter teeth. The answer is no.4,13-16 This answer is based on the fact that there is no evidence to prove that the peroxide in whiteners causes cancer. All of the credible scientific evidence that does exist indicates that there is no association between hydrogen peroxide and cancer.13-16

What’s more, when tooth whitening treatments are performed correctly, the exposure to hydrogen peroxide is minimal. With a normal custom tray, the patient’s exposure is usually < 4 mg of hydrogen peroxide per treatment. The oral cavity can quickly eliminate 30 mg of hydrogen peroxide within 1 minute.17

Overall, the evidence available at this time indicates no significant risks for cancer or cancer-promoting activities by using peroxide-containing tooth whitening treatments.

2. Is the Use of Potassium Nitrate Dangerous?
Potassium nitrate is used in dentistry to combat tooth sensitivity during the tooth whitening process, either separately or in the whitening gel.5,18,19 Approximately 6 years ago, brochures were distributed throughout the profession claiming that peroxide whitening gels that contained potassium nitrate would increase a patient’s risk of cancer. However, when queried, the distributor could not provide evidence to substantiate their statements.

According to conversations the author has had with the experts at the National Academy of Sciences and National Toxicology Program concerning potassium nitrate and potassium nitrite, all data available thus far indicate that there is no evidence to suggest that potassium nitrate increases cancer risk.

In addition, nitrate and nitrite are two different chemicals. Nitrite is more closely associated with a conversion to nitrosamines. Nitrate must be converted to nitrite to then form nitrosamines. This is possible only under certain conditions. Whitening gels containing peroxide actually promote the oxidation process. Therefore, the nitrate could not be converted into nitrite during the whitening treatment. Consequently, the answer to this question is no.5

3. What Effects Does Tooth Whitening Have on Tooth Structure (ie, Enamel/Pulp), the Soft Tissues, and/or Dental Materials?
It is generally accepted that tooth whitening is associated with tooth sensitivity and gingival irritation.20 However, these effects are usually mild to moderate and transient, disappearing when the patient completes or stops the whitening procedure.

Greater negative side effects can occur when products are used improperly or are of poor quality.5 For example, during clinical research studies conducted at Loma Linda University, patients were encountered who used assigned whiteners excessively to whiten their teeth quickly (eg, as many as seven applications a day for several days) disregarding the clear instructions for twice-daily applications. Tooth sensitivity and gingival irritation observed in such cases are not the normal response.

Enamel
Many studies have been reported in the literature regarding the effects of tooth whitening on enamel. The effects can be divided into three categories: reduction in enamel surface microhardness (some studies indicate there is a negative affect and others say there is not21,22); effects on enamel surface morphology, which have been shown to be quite different depending upon the types of products used23-28; and enamel loss,29 which has been reported mostly in several in vitro studies and in two case reports. One such case in 1991 involved a 13-year-old boy who presented unable to eat as a result of tooth sensitivity. An examination revealed that the majority of the enamel on his anterior teeth was gone and the dentin was exposed. It was found that the boy had purchased an over-the-counter whitening kit and used it excessively. At that time, some whitening products were fairly acidic, and a low pH is known to contribute to the loss of enamel minerals.

Pulp
In the early years of tooth whitening, scientists did not know how—or how much—peroxide from the whitening gel would enter into the pulp chamber. Today, it is known and accepted that hydrogen peroxide diffuses through tooth structure into the pulp, where it can be detected after some time. During its diffusion, hydrogen peroxide interacts with pigment molecules, causing the whitening effect.30-32 Although this mechanism is good for whitening efficacy, it has implications for possible pulp reactions.33-36

Investigators have shown that after applying different concentrations of whitening agents for several hours, different amounts of peroxide can be detected in the pulp chamber.37-40 The amounts detected appear to be higher when a higher concentration of gel is used, but not much higher. This may explain why gels of higher concentrations of peroxide usually produce greater tooth sensitivity.

Soft Tissues
The reported incidents of gingival irritation range from very low to fairly high in most studies, but the effects reported are dependent upon the methodology, regimen, whitening treatment, and concentration of the whitening gel (ie, the higher the concentration of gel, the higher the incidence of gingival irritation).5,20,41,42 Similar to tooth sensitivity, gingival irritation occurs in the early stages of the whitening process and has often been associated with an ill-fitting custom tray. Once the tray has been trimmed and/or the whitening procedure is finished, most gingival irritation problems resolve themselves.

Restorations/Bonding
Tooth whitening has an inhibitory effect on bonding.43-45 Therefore, clinicians must be careful when they perform whitening in relation to when they perform restorative bonding. Studies have advocated a waiting period ranging from at least 24 hours to a minimum of 4 days to 2 weeks before bonding restorations. This author recommends waiting a minimum of 2 weeks to ensure that there will be no detrimental effects on bond strength.44

Additionally, studies have shown that tooth whitening affects the surface roughness and gloss of existing restorations.46 However, the effects are dependent upon the material studied. One thing was made clear: Whitening does affect different resin materials in terms of color change, but it is not a whitening effect and it is not as evident a color change as seen with enamel.47,48

4. How Safe are Whiteners Containing 10% Carbamide Peroxide or Equivalently More than 3.5% Hydrogen Peroxide?
A vast amount of data is available to support the safety of whitening gels based on 10% carbamide peroxide. For those that contain higher concentrations of whitening ingredients, research indicates greater tooth sensitivity and gingival irritation.49,50 Some in-office products currently contain as high as 38% hydrogen peroxide. The lower end is 15%. Over-the-counter (OTC) products range from 3% hydrogen peroxide to 14%. There are limited data to indicate that gels of 15% carbamide peroxide, which is approximately 5.25% hydrogen peroxide, are only slightly more toxic than the 10% carbamide peroxide gels. Although most of these formulations with > 10% carbamide peroxide appear safe (because so far there have been no reports suggesting significant adverse effects), the data on them are still limited. Therefore, clinicians are advised to be cautious. All options and their potential risks should be presented to patients so they can make an informed decision.

5. Are Over-the-Counter Whiteners Safe? 
To date, there are little data regarding the safety of currently available OTC products. Most of them appear safe, although some products have been shown to have a significant negative effect on enamel.51,52 Therefore, when consulting with patients who express an interest in pursuing OTC whitening regimens, it is important for the dental professional to first provide a correct diagnosis of the discoloration problem to help reduce the potential risks that may be associated with the inappropriate use or abuse of the whitening products.

6. How Valuable is the Seal of the American Dental Association?
There are different points of view regarding the value of the American Dental Association (ADA) Seal of Acceptance. Obtaining the ADA Seal of Acceptance does require products to undergo extensive safety and efficacy evaluations. Therefore, should any problems arise, professionals who use a product that has been given the ADA Seal are in a much better position to defend their product choices than if they used a product without it.

The ADA has announced that its Seal of Acceptance program for professional products will be phased out by the end of 2007. The consumer product part of this program will be further enhanced. At the same time, efforts are being made to establish specifications that will serve as the national (American National Standard/American Dental Association [ANSI/ADA]) and international (International Organization for Standardization [ISO]) standards for tooth whitening materials. These standards will provide guidelines and requirements on adequate scientific evidence on the safety of tooth whitening materials.

Conclusion

Overall, research indicates that tooth whitening is safe with no long-term negative effects to the tooth structure or the soft tissue or to the systemic health of the patient. When used properly, dentist-prescribed tooth whitening products are effective and well-tolerated, and the most commonly seen side effects are transient tooth sensitivity and gingival irritation. However, significant adverse effects may occur with abuse or using products of poor quality.5 Because they can properly advise patients as to appropriate use and anticipate/diagnose any existing and potential problems, the risks that may be associated with tooth whitening can be minimized when whitening systems are dispensed by clinicians.53

References

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22. Lopes GC, Bonissoni L, Baratieri LN, et al. Effect of bleaching agents on the hardness and morphology of enamel. J Esthet Restor Dent. 2002;14(1): 24-30.

23. Haywood VB, Leech T, Heymann HO, et al. Nightguard vital bleaching: effects on enamel surface texture and diffusion. Quintessence Int. 1990;21(10):801-804.

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27. Ernst CP, Marroquin BB, Willershausen-Zonnchen B. Effects of hydrogen peroxide-containing bleaching agents on the morphology of human enamel. Quintessence Int. 1996;27(1):53-56.

28. Zalkind M, Arwaz JR, Goldman A, et al. Surface morphology changes in human enamel, dentin and cementum following bleaching: a scanning electron microscopy study. Endod Dent Traumatol. 1996;12(2):82-88.

29. McCracken MS, Haywood VB. Demineralization effects of 10 percent carbamide peroxide. J Dent. 1996;24(6):396-398.

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33. Cohen SC. Human pulpal response to bleaching procedures on vital teeth. J Endod. 1979;5(5):134-138.

34. Robertson WD, Melfi RC. Pulpal response to vital bleaching procedures. J Endod. 1980;6(7): 645-649.

35. Bowles WH, Thompson LR. Vital bleaching: the effects of heat and hydrogen peroxide on pulpal enzymes. J Endond. 1986;12(3): 108-112.

36. Seale NS, McIntosh JE, Taylor AN. Pulpal reaction to bleaching of teeth in dogs. J Dent Res. 1981;60(5):948-953.

37. Bowles WH, Ugwuneri Z. Pulp chamber penetration by hydrogen peroxide following vital bleaching procedures. J Endod. 1987;13(8): 375-377.

38. Jorgensen MG, Carroll WB. Incidence of tooth sensitivity after home whitening treatment. J Am Dent Assoc. 2002;133(8):1076-1082.

39. Slezak B, Santarpia P, Xu T, et al. Safety profile of a new liquid whitening gel. Compend Contin Educ Dent. 2002;23 (11 suppl 1):4-11.

40. Pohjola RM, Browning WD, Hackman ST, et al. Sensitivity and tooth whitening agents. J Esthet Restor Dent. 2002;14(2):85-91.

41. Curtis JW, Dickinson GL, Downey MC, et al. Assessing the effects of 10 percent carbamide peroxide on oral soft tissues. J Am Dent Assoc. 1996;127(8):1218-1223.

42. Leonard RH, Sharma A, Haywood VB. Use of different concentrations of carbamide peroxide for bleaching teeth: an in vitro study. Quintessence Int. 1998;29(8):503-507.

43. Sundfeld RH, Briso AL, DeSa PM, et al. Effect of time interval between bleaching and bonding on tag formation. Bull Tokyo Dent Coll. 2005;46(1-2):1-6.

44. Cavalli V, Reis AF, Giannini M, et al. The effect of elapsed time following bleaching on enamel bond strength of resin composite. Oper Dent. 2001;26(6): 597-602.

45. Josey AL, Meyers IA, Romaniuk K, et al. The effect of a vital bleaching technique on enamel surface morphology and the bonding of composite resin to enamel. J Oral Rehabil. 1996;23(4):244-250.

46. Yalcin F, Gurgan S. Effect of two different bleaching regimens on the gloss of tooth colored restorative materials. Dent Mater. 2005;21(5): 464-468.

47. Kim JH, Lee YK, Lim BS, et al. Effect of tooth-whitening strips and films on changes in color and surface roughness of resin composites. Clin Oral Investig. 2004;8(3): 118-122.

48. Kwon YH, Kwon TY, Kim HI, et al. The effect of 30% hydrogen peroxide on the color of compomers. J Biomed Mater Res B Appl Biomater. 2003;66(1):306-310.

49. Deliperi S, Bardwell DN, Papathanasiou A. Clinical evaluation of a combined in-office and take-home bleaching system. J Am Dent Assoc. 2004;135(5):628-634.

50. Li Y, Lee SS, Cartwright SL, et al. Comparison of clinical efficacy and safety of three professional at-home tooth whitening systems. Compend Contin Educ Dent. 2003;24(5): 357-364.

51. Cubbon T, Ore D. Hard tissue and home tooth whiteners. CDS Rev. 1991;84(5):32-35.

52. Hammel S. Do-it-yourself tooth whitening is risky. US News and World Report. April 2, 1998:66.

53. Haywood VB. Historical development of whiteners: clinical safety and efficacy. Dent Update. 1997;24(3):98-104.

About the Author
Yiming Li, DDS, MSD, PhD
School of Dentistry
Loma Linda University
Loma Linda, California

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