Are whitening products more effective on teeth with stains of certain colors?
Yes. Yellowish teeth respond well to bleaching, brownish-colored teeth respond less well, and grayish-hue or purple-stained teeth may not respond well to bleaching at all. Blue-gray staining caused by tetracycline is more difficult to lighten and may require up to 6 months of home treatments or several in-office appointments to successfully lighten. Teeth that have dark stains may be better candidates for another lightening option, such as veneers, bonding, or crowns. Your dentist can discuss the options best suited for your situation.

With all types of bleaching procedures, the degree of whiteness will vary from individual to individual depending on the condition of the teeth, nature of the stain, the concentration of the bleach and the duration of time and bleaching system used.

What follow-up is needed or is typical?
After in-office whitening, your dentist may want to see you in a few days to check on your gums. If whitening your teeth at home under a dentist-supervised process, your dentist may want to check your gums and your teeth whitening progress after about a week of applications and again a couple weeks later.

How often does the teeth whitening need to be touched up?
Whitening is not permanent. People who expose their teeth to a lot of foods and beverages that cause staining may see the whiteness start to fade in as little as 1 month. Those who avoid foods and beverages that stain may be able to wait 1 year or longer before another whitening treatment or touch-up is needed.

What can I do to maintain my brighter smile?
Tips for maintaining your newly whitened teeth include:

Avoiding the consumption of or exposure to products that stain your teeth (see first question in this document on what causes teeth to become stained). If you do choose to consume beverages that stain, consider using a straw so that the liquid bypasses your front teeth.

Brush or rinse immediately after consuming stain-causing beverages or foods.
Follow good oral hygiene practices. Brush your teeth at least twice daily and floss at least once daily to remove plaque. Use a whitening toothpaste (once or twice a week only) to remove surface stains and prevent yellowing. Use a regular toothpaste the rest of the time.
Consider touch-up treatments. Depending on the whitening method used, you may need a tough-up every 6 months or after a year or two. If you smoke or drink lots of stain-causing beverages, you may need a touch up more often.

Are there any risks associated with teeth whitening?
The two side effects that occur most often are a temporary increase in tooth sensitivity and mild irritation of the soft tissues of the mouth, particularly the gums. Tooth sensitivity often occurs during early stages of the bleaching treatment. Tissue irritation most commonly results from an ill-fitting mouthpiece tray rather than the tooth-bleaching agent. Both of these conditions usually are temporary and disappear within 1 to 3 days of stopping or completing treatment.

If you do experience sensitivity, you can reduce or eliminate it by:

Wearing the tray for a shorter period of time (for example, two 30-minute sessions versus two 60-minute sessions)
Stop whitening your teeth for two to three days to allow your teeth to adjust to the whitening proces.
Ask your dentist or pharmacist for a high fluoride-containing product, which can help remineralize your teeth. Apply the fluoride product to the tray and wear for 4 minutes prior to and following the whitening agent.
Brush your teeth with a toothpaste made for sensitive teeth. These toothpastes contain potassium nitrate, which helps soothe your teeth’s nerve endings.

Are these products safe and effective?
Some bleaching products dispensed through dentists’ offices as well as professionally applied (in-office) bleaching products have received the ADA Seal of Acceptance, which indicates that the product has meet ADA guidelines for safety and effectiveness. Currently, only dentist-dispensed home-use products containing 10% carbamide peroxide and office-applied products containing 35% hydrogen peroxide have received the ADA Seal of Acceptance. No over-the-counter products have received the Seal of Acceptance. Over-the-counter bleaching products are not endorsed by the ADA because the organization believes that professional consultation is important to ensuring safe and effective use. No whitening products using lasers currently are on the ADA’s list of accepted products. Several whitening toothpastes that are available over-the-counter have received the ADA Seal of Acceptance. For a list of specific toothpastes that have gained the ADA’s Seal of Acceptance, visit: http://author.my.clevelandclinic.org/Documents/HIC/adaseal_productlist.pdf (see dentifrice).

It should be noted that not all manufacturers seek the ADA’s Seal of Acceptance. This is a voluntary program that requires considerable expense and time on the part of a manufacturer. Just because a product does not have the ADA Seal of Acceptance does not necessarily mean that the product is not safe and effective. You can be assured, however, that products that do carry the seal have meet the ADA’s standards for safety and effectiveness when used as directed.

Teeth whiteners are not drugs and therefore are not regulated by the FDA.

Does insurance cover the cost of whitening procedures?
No. Dental insurance does not typically cost the cost of the tooth whitening procedure.

Do teeth whiteners damage tooth enamel?
Studies of whitening products using 10% carbamide peroxide showed little to no effect on the hardness or mineral content of a tooth’s enamel surface.

Do teeth whiteners damage existing dental restorations?
Over 10 years of clinical use of whitening products containing 10% carbamide peroxide have not shown any damage to existing fillings. The issue is not “damage” to existing restorations; rather, keep in mind that existing restorations such as tooth-colored fillings, crowns, bonding, veneers, and bridges do not lighten. This means that any preexisting dental work may need to be replaced to match the new tooth shade achieved in the natural teeth, should a bleaching process proceed.

Do teeth whiteners damage a tooth’s nerve?
There’s no evidence to date that the tooth whitening process has a harmful effect on the health of a tooth’s nerve. One study reported that at both a 4.5 and 7-year follow up, no individual who used a tooth whitening system needed a root canal procedure on any teeth that had been whitened.

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Source: https://my.clevelandclinic.org/health/articles/teeth-whitening/faqs