I learned about this many years ago and the difference after I started using only SLS-free toothpaste was night and day. I used to get canker sores (mouth ulcers) any time I would bite the inside of my cheek, hit my gums with the hard parts of my toothbrush, etc., and this completely stopped a while after I switched to SLS-free.
SLS is Sodium Lauryl Sulfate, by the way, and it’s a detergent. From what I understand, the only reason why it’s added to toothpaste is to make more foam when you brush. But the SLS-free toothpaste I use makes plenty of foam, so I have no idea why they add it. It’s one of those things about the modern world that makes absolutely no sense. The ads and packaging should say in big letters: “now with even more canker sores!”
Unfortunately, the vast majority of toothpastes on the market (at least in the US) have SLS. I can only seem to find SLS-free toothpaste in natural food/supplement stores. It’s extra difficult to find toothpastes that are SLS-free but that keep fluoride too. The difficulty (and price? I haven’t compared) is completely worth it to me though.
TL;DR: The SLS (Sodium Lauryl Sulfate) in most toothpastes is unnecessary and (edit: CAN cause) canker sores (painful sores in your mouth and gums). If you have this problem, you will likely benefit from SLS-free toothpaste (some still include fluoride) that you can usually find at natural food stores.
I used to get terrible stomach aches after brushing and it was enough that I completely stopped brushing for a long stupid time. Even when I made myself do it I ended up with this tons of skin falling off inside my mouth every morning if I brushed before bed. It was absolutely nasty. When the dentist used those plastic trays with the fluoride foam it would leave me curled up on the couch for the rest of the day in pain.
One day my wife suggested that maybe I have a problem with SLS and I switched to sensodyne pronamel which was the cheapest SLS free toothpaste I we could find and all of these problems went away. I’m now a regular brusher and feel stupid for not figuring it out years ago because my teeth have suffered from my ignorance.
In the interest of being thorough I switched back to my old toothpaste for a day and the symptoms immediately returned after just one night-time brushing. Sure I could go through many more changes to try to pin down exactly whether it is SLS or something else that’s different between the two toothpastes I was using, and I recognize that, but SLS was enough of a reason for me to switch and I’m happy now.