If you’re thinking of buying a teeth whitening kit that comes with a blue light because you saw Khloe Kardashian or other celebs hawking one on Instagram—or heard their merits sung on beauty blogs—don’t. After a week of reading some two dozen papers, interviewing three dentists, and trying one of the variety of kits that are out there myself, I’ve found there’s no evidence that lights will improve the appearance of your teeth in a lasting way—even those deployed by a professional or designed by a dentist. Though some lights can add a temporary boost that fades within a day or so, none ultimately make your teeth whiter than trays of bleach or whitening strips alone. Nor can they shorten the amount of time you have to spend with a mouth full of whitening chemicals to achieve your desired tooth shade. As one dentist who declined my interview request about the merits of lights said, “In my opinion, they’re BS.” How teeth whiteners work Despite the variety of kits and systems to choose from, there are two basic ways to whiten your teeth. You can physically scrape stains off of them, which is how many whitening toothpastes works. Or you can take them off chemically with bleach deployed in trays, paint-on gels, or strips, and sometimes in toothpastes. The do-it-at-home versions on Instagram tend to be on the inexpensive side and consist of basic bleach trays and LED lights, but there are kits on the market that go for upwards of $200, and are endorsed by practicing dentists. Lights are an add-on to the chemical stain removal process that supposedly either heat up the bleach or react with a photocatalyst in the gel to speed the chemical reaction. Lights that produce heat. These are prevalent in dentists’ offices. But Clifton Carey, a chemist at the University of Colorado who studies the mechanisms behind dentistry, says that in practice, there is simply no evidence that the light systems installed in so many dental practices across the country produce enough heat to make a difference in the speed of the reaction that is whatsoever noticeable or measurable. Study after study shows that if you put the same concentration of bleach on a patient’s teeth, sit them down for the same period of time, and shine a light on one group and not the other, they will ultimately wind up with the same difference in tooth color. If the lights sped up the reaction, you’d expect the group that got the light to have a greater change. And based on this kind of data, it seems that the light can’t simply reduce the amount of time you spend in the chair to achieve your desired results. In theory, dentists could just crank up the temperature to get a reaction that’s noticeably faster. But too much heat is bad for the soft innards of your teeth—the pulp, which consists of connective tissue and nerves. And though there’s no evidence that the available systems do damage to nerves, says Carey, there’s also probably no reason to risk being the first. As it stands now, the systems that produce heat do have a short-lived effect that’s separate from the bleach reaction. If you warm teeth, the enamel dries out and turns whiter, says David Sarrett, dean of dentistry at Virginia Commonwealth University. When you wipe off the bleach and first look in the mirror, the effect might leave you more satisfied with what you’ve been sold. But after the tooth rehydrates, the whiteness fades and your teeth will be the same color it would be if you hadn’t used the light at all. (All tooth bleaching will dry out the teeth a little, but the effect will be more pronounced if there’s heat involved.) Because any tooth whitening process can make your teeth sensitive, it’s not advisable to whiten your teeth right before a big meeting or date, even to get the extra temporary boost of whitening. Photocatalyst-based systems. These use lights that do not generate significant heat. These include the at-home kit IntelliWhite Cool Blue Pro, or the Zoom in-office system. They claim to instead rely on a solution containing a photocatalyst, a chemical that will speed up a reaction when hit with light, according to Jennifer Jablow, a dentist and the creator of the IntelliWhite. Jablow claims her system makes teeth five shades whiter after five treatments of five minutes each; Crest Whitestrips claim you’ll start noticing results after 90 total minutes of use. It’s possible that (mostly) heat-free lights work, says Carey. But there are no publicly available studies that he or I could find to confirm this. When I asked Jablow for data, she said she has completed independent clinical studies showing the effectiveness of her product but she could not share them with us, which does not do us any good. And anecdotal evidence from user reviews suggests that Jablow’s claim that her product whitens teeth by five shades is overblown. “Where are the white teeth????” asks one. Though some users were satisfied, the product has 2½ stars (out of five) across 37 reviews on the Home Shopping Network. So, if they don’t offer any lasting benefits, why do dentists sometimes use lights during the teeth whitening process? Well, they’re showy, for one. From spending two days trying out the GLO Science, I can attest to the fact that having a warm blue mouthful of bleach feels more high-tech and official than having a couple bumpy slimy white strips affixed to your teeth. Lights are also just what patients who favor in-office whitening have come to expect. Light-based whitening systems have been around dentists’ offices for decades, since before researchers started studying how teeth whitening actually works, says Sarrett. If dentists stop using the lights, patients may think the system is subpar. One owner of a company that makes whitening systems that do not use lights told me that he sends clients a script to use to convince patients that it still works: He tells dentists to lie and say they have both systems, but that they’ve been finding that the lightless system works even better. (There’s no evidence that this is true, either; although, again, there is a potential risk in heating up your teeth.) Companies that sell kits with lights are at least somewhat aware of the doubt that surrounds them. When I called in a GLO Science kit for testing, the assistant who took down my address told me that frankly, many of the kits out there were gimmicks—only kits like the GLO Science that use heat truly work. And when I emailed with Jablow, the creator of the IntelliWhite, which uses LEDs, she told me to steer clear of heat-based systems because of their potential to damage teeth. “Our light is the only one on the market that actually works,” she said. So, what works? Sarrett and Carey both say that there are just two factors in getting good results with a chemical whitening kit: the amount of bleach you use and the amount of time it’s sitting there. One session in the dentist’s chair isn’t typically enough to achieve the desired result, which is frustrating for patients. In many cases, teeth become too sensitive from the bleach before they’re as white as the patient wants them to be. In an effort to defy that reality, says Sarrett, “I think a lot of people are looking for a magic bullet.” The GLO Science kit’s creators claim that the process does not make your teeth sensitive, which would be a benefit over other bleaching systems. But I found that I could make it through only 16 minutes of the recommended 32 on the days I tried it before my teeth began to feel tender enough that I didn’t want to continue.