I just discovered Pawbreakers last week. I didn't buy any but I'd be interested to know what's in them too. They seem to want to keep it a secret. You could always email them and ask if they'd send you an ingredients list, although I'm guessing they won't want their secret to get out.
They reassure several times that their ingredients are safe, natural and human grade. I'd be inclined to trust them.
After reading the website, I think I might get some, they look like fun.
"One cat just leads to another." - Ernest Hemingway
Meowy Meowers - Angus n' Finn - 5 yrs old
Barky Barker - Skylar - 4 yrs old??