Looks like a lot of people blister their thumbs but I don't know of anyone and I have never ran into this problem. Could it be cheap grips. Either way this product did nothing for me and didn't add any cool factor to my bike.
Got these for my daughters Honda 150 CRF. The stock grips rubbed her knuckles something fierce, both hands. I got these and slipped them on. They created a nice soft friction less rub for the thumb knuckle.