There are two types of whitening products you can use at home, those that actually "bleach" the teeth by penetrating the transparent enamel and lightening the layer underneath, or those that remove surface stains.
The efficacy of both types of treatment will depend on in what manner and how deeply your teeth are stained, but generally a good rule to keep in mind is that the longer the whitener stays in contact with your teeth, the better the results will be.
For this reason, whitening mouthwashes tend to be the least effective of all at-home treatments, although as a part of an overall whitening plan, they certainly can't hurt.
Next up the line comes whitening chewing gum. One study showed that gums containing a substance known as sodium hexametaphosphate (like Orbit) actually helped reduce new stains by 33 percent [source: Porciani]. Additionally, chewing sugar-free gum is now generally regarded as a smart part of a dental hygiene program, as it helps produce saliva, which kills decay-causing bacteria.
Whitening toothpastes are more effective than both mouthwash and gum at removing stains from teeth -- especially if you follow the American Dental Association's recommended guideline of brushing for two minutes twice a day. But it's important to note that toothpastes are effective at lightening or removing surface stains, not actually changing the color of the dentin that lies beneath the enamel and gives our teeth their characteristic hue.
Whitening toothpastes work either through gentle abrasives or through the use of chemicals such as sodium tripolyphosphate that help break down stains. It typically takes about two to four weeks for whitening toothpastes to take effect when they're used twice daily for two minutes. However, there is research showing that a chemical known as blue covarine can make teeth look instantly whiter by adhering to teeth and creating an optical illusion that makes them seem less yellow [source: Carr]. It's only available in Closeup White Now toothpaste, and not yet in the U.S. as of this writing. But buyer beware: Like any new product, it seems to be getting mixed reviews -- some users rave about its whitening abilities, while others, well, aren't so happy with their results.