NEW YORK – From toothpastes and gels to mouth trays and rinses, there is a dizzying array of products that claim to lighten, brighten and whiten your teeth. And with so many options lining pharmacy shelves, it can be daunting to figure out the right method for you.

But are at-home teeth whitening products as effective as they claim? And are they safe? Here is what some experts say.

How does teeth whitening work?

Teeth whitening products sold over the counter work in one of two ways. They either scrape away stains using physical force or bleach those stains with the same chemicals used for in-office whitening procedures.

Physical abrasion

Products that have any sort of grittiness – whether they are whitening toothpastes, regular toothpastes or just plain baking soda – will act as an abrasive and physically file away stains that occur on the surfaces of your teeth, said Dr Dorota Kopycka-Kedzierawski, a dentist at the University of Rochester Medical Center. Many regular toothpastes contain a little bit of texture for this reason, and brushing itself is an act of physically scraping off stains and debris.

Chemical bleaching

Most whitening strips, gel-filled mouth trays and rinses work by putting your teeth in contact with chemical bleaching agents such as carbamide peroxide or hydrogen peroxide. One major difference between these products and in-office versions, said Professor Timothy Bromage, is that the concentrations of chemicals in over-the-counter products are much lower. He is a professor of molecular pathobiology at the NYU College of Dentistry who specialises in bone and tooth biology.

Which products work?

Physical abrasion and chemical bleaching products are both effective at whitening teeth, said Professor Laura Tam, dean of dentistry at the University of Toronto.

When toothpaste is advertised as “teeth whitening”, it usually means it has more of an abrasive quality than regular toothpaste, she said, but even regular toothpastes that do not have any “whitening” claims on their packaging will work.

Chemical bleaching products can similarly reduce external stains — whether from coffee, tea, red wine or cigarettes. They can also be effective at reducing stains that occur in the inner layer, or dentin, of the teeth, which can develop after blunt force or traumatic tooth injuries, or in children after taking certain antibiotics, Dr Kopycka-Kedzierawski said.

Pigmented foods and drinks can also seep into the enamel and stain the insides of teeth. Some people with naturally very porous enamel are especially prone to this kind of staining.

The only trade-off with at-home chemical bleaching products is that since the concentrations of chemicals are much lower, they will take longer to work than in-office methods, Prof Tam said. And not everyone will see whitening effects on their internal teeth stains. People’s enamel will absorb bleaching agents differently, she said, so the efficacy can vary. And some people just have naturally darker teeth than others because of their genetics.

LED light whitening kits are another popular product. They involve applying a whitening gel to your teeth and holding a tray in your mouth, or biting into a mouth tray full of gel, and using a bright light to illuminate the gel.

In theory, they work because LED light can activate the bleaching chemicals to get them to work faster, Prof Tam said. Unfortunately, she added: “The body of evidence does not suggest you can activate the bleaching agent using anything like light or heat.”

Because these products involve applying bleaching chemicals to your teeth, you might still see whitening results, but the LED light process is probably not adding much.

Baking soda, mixed with water to form a paste, is also something Dr Kopycka-Kedzierawski said her patients had commonly used as a scrub for the teeth. There is evidence it can work to remove external stains and reduce bacteria, she said, although you are probably better off just using a dentist-recommended toothpaste, since baking soda does not provide fluoride.

Activated charcoal is another ingredient often included in whitening toothpastes. However, there is no scientific evidence that activated charcoal is effective or safe for dental care, and the American Dental Association does not recommend it, Dr Kopycka-Kedzierawski said. In fact, activated charcoal can be overly abrasive and can damage your enamel and gums.

Are teeth whitening products safe?

Abrasive teeth whitening products are generally safe, Prof Tam said, but scrubbing your teeth with particles that are too hard, or brushing with abrasive material for longer or more often than is recommended, can wear down the enamel.

This can be counterproductive to your whitening efforts, Prof Bromage said, since the enamel is what that makes your teeth look white. Dentin, the stuff that makes up the inner part of the tooth, is a yellow-brown color. If you wear down the white enamel too much, the colour of the dentin starts to show through.

In general, Prof Tam said, at-home chemical bleaching treatments are safe and effective when used as advertised. But it is best to go with products that have lower concentrations of chemicals and shorter exposure times, she said. Using chemical-based treatments incorrectly, by leaving them on for too long or applying the chemicals too high so they reach your gums can put you in danger of damaging your teeth and injuring your gums.

In this vein, Prof Bromage said, over-the-counter whitening products should not hurt, especially on first application. Pain is a sign that you are either overdoing it or that there is something happening that requires your dentist’s attention.

Anything that claims to be “extra strong” is probably best avoided, Prof Tam said.

The bottom line

Teeth whitening is never a permanent fix, Prof Tam said. At-home results can last from a few weeks to a few months. (In-office whitening procedures can last for a year or more.) And because teeth naturally darken with age, she said, you will see regression no matter which treatment you opt for.

It is also important to keep in mind that none of these whitening procedures will work on fillings, crowns or other dental hardware, Dr Kopycka-Kedzierawski said. Whitening will lighten your natural teeth, which could create unevenness if your fillings are darker. In those cases, your dentist can recolour those mismatches by painting over them with matching shades.

In the end, she said, everybody’s teeth are different. The natural colour of your teeth, the porosity of your enamel and the sensitivity of your teeth can make you react differently to whitening treatments. When in doubt, check with your dentist. NYTIMES