"Cristiano Ronaldo got paid mad money to do that ad, and you shouldn't really be using these things, because the masseter muscles are really strong, but the reality is that if you overdo it, you can create problems in the TMJ area that literally, literally causes you to have a pain in the head," he says.
"And then at night, if you overtrain these muscles, you can start doing teeth clenching and teeth grinding, wearing down your molars," he continues.
"This is one of those areas where there really isn't a lot of evidence, good clinical evidence, to support it.