Walk down the toothpaste aisle at your typical drugstore and you’ll see a range of products that promise to whiten your teeth. Whitening toothpaste, whitening strips, a whitening gel that you can paint on your teeth with a cotton swab or use in a mouth tray, a two-step “daily cleaning and whitening system,” and more.

“I see a lot more attention on pretty smiles,” says Clifton Carey, a chemist at the University of Colorado’s School of Dental Medicine. Tooth whitening, in particular, is “a big thing these days. A lot of sellers and a lot of customers.”

The products at the drugstore all have essentially the same whitening ingredient — the bleaching agent peroxide. If you go to your dentist’s office for a professional tooth whitening, it’ll use a more concentrated peroxide product.

With the in-office procedure, “you get a lot of whitening very quickly, but it requires expertise,” says Matthew Messina, a practicing dentist at the Ohio State University College of Dentistry and a spokesman for the American Dental Association. With such a high-powered bleaching agent, he says, “the dentist has to protect the gums.”

The over-the-counter products are weaker. That means less active whitening but also less risk to the gums, should the whitening agent come in contact. “All of the products are safe if used as directed,” Messina says. Still, they can increase sensitivity of teeth and they can irritate gum tissue. “Anything that doesn’t feel right, you should see your dentist.”

Tooth whitening is best done in a “healthy mouth condition,” Messina says. “Have a thorough exam, make sure your teeth are clean and that plaque and tartar have been removed.” Also, be aware that tooth whitening doesn’t work on crowns or most fillings.

Professional whitening, which will be immediate and last for years, might cost $500 or more and is not generally covered by dental insurance. Products to use at home usually require multiple applications over a week or two, will have a gradual and lesser whitening effect, and will not last as long. Whitening strips can cost as little as $25.

“The do-it-yourself products can be used as a booster, after a professional treatment, to keep the teeth white,” Carey says. “Dentists often recommend this.”

How do these products work? “It’s a surface-type bleach,” Carey says, working on stains that are bonded to tooth enamel. “Bleach is a chemical that breaks those bonds,” Carey says. The staining compounds might remain, but the bleach turns them clear.

The concentrated product that dentists use also dehydrate the tooth somewhat. “That’s the immediate color change — bleaching plus dehydration,” Carey says. As the surface of the tooth rehydrates over the next few weeks, people may notice their teeth’s whiteness slip back a couple of shades.

A quick bit of tooth anatomy: Enamel is the thin hard outer layer of the tooth; dentin is the next layer in and is less dense than enamel. In the middle is the pulp, which is the soft tissue that holds the nerve center.

“Enamel is what you’re bleaching,” Carey says. “It’s really thin near the gum line. If you have receding gums, it exposes your root tissues.” Dentists try to avoid applying the concentrated product on or near the dentin.

Dentin comes into play for aesthetics as well, because it’s got a naturally yellowish hue. As people age, their enamel can become thinner, a result of decades of wear and tear. The thinner the enamel, the more likely the yellowish dentin shows through. That’s why older people often have yellowed teeth. Bleaching products won’t help in this situation because they don’t affect the dentin.

There are a few other discolorations that whitening procedures cannot change. If you took tetracycline as a kid, say for an ear infection, you might have antibiotic staining of the teeth. Or if your teeth suffered trauma when you were young and your enamel was still forming, whitening won’t work. Dentists refer to these discolorations as intrinsic stains.

If you search online, you’ll find plenty of ideas about natural methods of tooth whitening. Use lemon juice or apple cider vinegar as a mouthwash? Scrub your teeth with an activated charcoal product? Messina says those techniques come with risks. The acid of lemon juice can erode the enamel on your teeth, and charcoal is an abrasive that can wear it away. “Your teeth will be whiter initially, but as the enamel wears away, you’ll see more dentin,” Messina says. “That yellowish color will show through.”

Can you prevent your teeth from becoming stained in the first place? Good practices mean avoiding staining substances. The most egregious are red wine, coffee and tobacco residue. Other foods on the staining list are tea, tomato sauce and balsamic vinegar. If the thought of avoiding any of those makes you want to cry, Messina advises rinsing with water after eating. Brushing your teeth is even better.

And, of course, the dentists advise good oral hygiene: regular brushing, flossing and checkups.