What is the ingredient in toothpaste that causes canker sores

Photographed by Erin Yamagata.

I arrived home from a recent trip to Tadoussac, a tiny village three hours north of Quebec best known for its whale-watching, named for the Innu word for "boobs," with a belly full of airport poutine, a shaky iPhone video of an enormous humpback breaching right in front of our Zodiac boat, a souvenir T-shirt depicting several different species of baleine, and two increasingly painful canker sores, too irritating to ignore.

What had I done differently in that True North, I wondered, other than eat a type of unpasteurized cheese that is illegal in the United States? The answer came to me one night days later, as I spat a mouthful of gray foam — the lather from my go-to charcoal-laced natural toothpaste — into my own bathroom sink.

Because I had chosen not to check a bag for the short sojourn to Canada, being that I am so very low-maintenance, I'd requested that my boyfriend pack his travel-size toothpaste for us both. For four days and three nights I used this toothpaste, from a popular drugstore brand with which many Americans trust their teeth, once in the morning and once before bed. But using normal toothpaste like a normal American does not suit me, which reminded me of why I stopped using normal toothpaste in the first place, all those years ago: Sodium lauryl sulfate gives me, and plenty of other people, huge fucking canker sores.

"I've seen many patients with painful canker sores and mouth ulcers caused by toothpastes containing sodium lauryl sulfate (SLS)," says dentist Ramin Tabib, D.D.S., of NYC Smile Design. "People come to us on an emergency basis worried about the sores in their mouth, and the first question we ask is whether they've switched toothpastes recently." There are a couple other ingredients in toothpaste known to cause irritation — namely pyrophosphates, which are used to control tartar, and added flavoring ingredients, to make sure you get that minty taste you like — but it's the SLS that will really knock you down and make you beg for Orajel.

As Dr. Tabib explains, the strong surfactant's foaming action damages the skin's proteins and dries out the inside of your mouth, making it a severe irritant for many people. And yet you'll still find it in just about every toothpaste at the drugstore, a canker-sore catastrophe free with purchase. "The drive in keeping SLS in toothpaste is that it's manufactured inexpensively, and that the lather is something the consumer looks for and associates with 'squeaky clean,'" he says. "They are correct in that it effectively produces squeaky-clean skin surfaces, but it works too well."

Dr. Tabib recommends Jason toothpaste and Sensodyne ProNamel; I'm a fan of Schmidt's, and the fact that I am not alone. Though perhaps I'd like to be, steering a tiny boat over that big, dark expanse of water where the Saguenay and the St. Lawrence rivers meet, with my fate in the hands of the tide and the god I don't believe in, while the endangered blues swim beneath me and the belugas come up to whistle and breathe. I will bring my own toothpaste this time.