If you have a crooked front tooth, they fit because of the molding bars. you're best off wearing one of the two.
If you wear dentures, you have to shape the molds to your teeth in order to wear them. The prongs in the back can be adjusted which may allow it to work.
If you feel loose-fitting, PLS search for a quick video showing how to mold them. If it is still loose fitting, please send it back
If you feel bad, please take a couple of hours to get used to it. Varies from person to person.
If you have a question about if it fits everyone. If the teeth are average or slightly bigger than average it should fit but if it's a slightly smaller than average mouth it maybe is a little too big for them. You can bend the Grillz to fit your mouth, so that helps it fit a larger range of teeth.
NOTE: This product is made of metal. Do not bend it when the temperature is very low