Botox is very effective in slimming the jaw line or face if the masseter muscle (the main muscle used for chewing) is the cause of bulkiness along the jaw line. People who clench and grind their teeth at night develop a strong, square jaw line from over usage of this muscle. Through injections into the masseter muscle, Botox can create a more feminine, slim appearance to the jaw line and face. You can expect your Botox to last up to six months in this area. You will start noticing a reduction in your jaw line a few days to weeks after treatment, with a significant reduction after 1-2 months.