Pearly whites gone yellow or brown? Brightening them can create a more polished first impression and even take a few years off your appearance. But which whitening options work best — and how safe are they, really? Brush up on the facts.

Before making a decision, talk to your dentist. Your teeth, and how they got stained, may dictate what treatment will work best for you.

“Individuals with yellow tones to their teeth respond best to bleaching,” says Hans Malmstrom, DDS, director of advanced education in general dentistry at the Eastman Institute for Oral Health at the University of Rochester Medical Center. Malmstrom conducts clinical trials in dental implants, adhesives, cements and bleaching.

“Brown-tinted teeth bleach less well, and grayish teeth — sometimes caused by tetracycline antibiotic use during childhood — will require significantly longer exposure to bleaching agents,” he says. In these cases, daily bleaching for six months or longer under the supervision of a dentist may be necessary, and even then, there’s no guarantee of success.

Another factor to consider is the amount of dental work in your mouth. “Whitening only works on teeth. Porcelain crowns, bridges or fillings won’t be affected by whiteners and may become more noticeable afterwards when they are surrounded by whiter teeth,” cautions Malmstrom.

Your age and the strength and porousness of your tooth enamel also factor in to how your teeth react to whitening.

Related: 7 Surprising Ways to Brush Your Teeth Better

Over-the-counter strips and trays

These products will probably whiten your teeth, maybe in as little as two weeks, but don’t expect the same results you’d get from professional whitening, says Malmstrom. In some cases you also might not see the desired whitening effect before you’ve reached the manufacturer’s limit on recommended uses.

What they cost: One of the most popular whitening strips, Crest 3-D White Whitestrips, sells 14 strips (to be used once daily) for $30. Shine Whitening Kit offers a tray and bleaching gel that you use for 10 days and it retails for around $40.

Who should consider them: “Teeth whitening using an OTC product is ideal for people who have healthy teeth and gums, few fillings and other dental work,” says Malmstrom. According to the Cleveland Clinic, the most common side effects are a temporary increase in tooth sensitivity and irritation of the soft tissues of the mouth, particularly the gums.

Malmstrom cautions people with sensitive teeth, exposed roots, unfilled cavities, broken fillings, cracks or loose dental work to not use OTC teeth whitening products. “If you have a crack in your tooth or a leaking filling for example, the bleaching solution can get inside the tooth, irritate the pulp and possibly cause the nerve to become inflamed which can lead to an abscess,” says Malmstrom.

How safe are they? OTC whitening agents are classified as “cosmetics,” and therefore are not regulated by the Food and Drug Administration (FDA). “The truth is a lot of these products haven’t been widely studied,” says Malmstrom, adding that use by children has also not been sufficiently studied.

A product containing 10 percent carbamide peroxide (the active ingredient) is the only concentration that has been well researched and approved by the American Dental Association (ADA), notes Malmstrom. Currently, the ADA has only given its seal of approval for safety and effectiveness to Opalescence Whitening Gel 10% (Ultradense Products, Inc.)

Stronger non-ADA approved products are available. However, because manufacturers are not required to list the percentage of carbamide peroxide in a product — and the treatment is not supervised by a professional — you risk damaging your gums and teeth when using one of these, says Malmstrom. 

There are also unanswered questions about the general safety of OTC whiteners. The ADA officially appealed to the FDA to regulate these products because of concerns about safety. In an April 2014 statement to the FDA the ADA wrote, “The exact mechanism of peroxide-containing tooth whitening is not well-understood, making it difficult to determine whether the various formulations contained in the marketed products affect the structure or function of the body.” 

What else to know: Don’t overdo it. “Bleachorexia” — addiction to tooth whitening — can backfire. “It’s possible to do real damage, especially through overuse by people attempting to achieve results faster,” warns Malmstrom. Follow the instructions for how often the product can safely be used.

Dentist-supervised home (DSH) bleaching

Patients are fitted with custom-made trays and provided with a bleaching gel recommended by the dentist. Unlike strips, this technique bleaches the whole mouth.

What it costs: The cost averages $300 to $600 for the whole mouth; to save money some people opt to whiten the top teeth only, which cuts the price in half. 

Who should consider it: Anyone who doesn’t want to risk gum irritation, especially people with sensitive teeth. Because the tray fits better than the OTC tray, there is less leaking of the bleach solution and therefore less risk of gum irritation. 

How safe is it? This is the ADA’s preferred approach to whitening because a professional is monitoring the process. And in cases where a higher concentration of bleach is prescribed (based on the amount of whitening you may need), you benefit by being having the dentist monitor how your gums and teeth react.

What else to know: DSH bleaching takes about two weeks of daily use to see results.

Related: Are You Neglecting Your Dog's Teeth? 

In-office whitening

This procedure involves one 90-minute session. A trained technician or dentist applies a layer of bleaching gel (typically a higher concentration of carbamide peroxide) than what’s available OTC. The patient sits with the gel for several minutes, then the technician wipes off the bleach, allows the teeth to rest, then starts again.

What it costs: The procedure averages between $500 and $1000 per session, depending in part on the extent of the discoloration.

Who should consider it: People who need more aggressive whitening or want fast results. At the end of the session, teeth are typically six to eight shades lighter. People with sensitive may not be able to endure three rounds of the application. However, a dentist can numb the teeth and gums with a local anesthetic before starting the treatment.

How safe is it? Because the dentist uses a higher concentration the results are more dramatic. But this also means that some patients may experience uncomfortable stinging and temporary numbness.

What else to know: One of the most widely used in-office procedures in the United States is Zoom. Since whitening is not permanent, patients also receive a custom-fit tooth tray and whitening gel for periodic home use to maintain the results.

Ann Matturro Gault is a freelance writer and editor whose work has appeared in national magazines and many websites. She lives with her four kids, dog, cat and spouse in New Jersey.