After jaw Botox, will the result get ruined if I use my jaw muscles right away and chew hard food like carrots?

After Botox for masseter reduction, you will be able to eat normally right away.  It may take 1-2 weeks to see your final results after your procedure. Best, Dr. Emer

While I don't believe that “exercising” your jaw by chewing hard foods such as carrots will make your Botox treatment less effective, I would advise you to avoid doing anything to build up the masseter muscle again. First, we do ask our patients to refrain from exercise for 24 hours after Botox  – no matter where they're treated. Second, giving your jaw a “workout” by eating difficult, chewy foods may build up that masseter muscle that you just used Botox to slim. Feel free to eat normally, but limit chewy bagels, dried sausage, jerky, gum and yes, hard carrots – anything that could begin building up the muscle again. Please see a board-certified dermatologist to have your jaw slimmed with Botox. 

Having BOTOX® injected into your masseter is a considered to be a safe way to minimize the prominence of your jaw. People are usually asked to avoid exercise immediately following a BOTOX® injection to prevent it from migrating to other areas of the face. This is not as much of a risk when it comes to the chin and jaw area. However, over time, a lot of chewing, particularly when it comes to very hard foods, could eventually cause the related muscles to enlarge.