Although temporary, BOTOX can be safely injected into the jaw muscle (masseter muscle) at intervals of three to six months.

Weakening the muscle's nerves leads to atrophy or shrinkage that may last for up to six months.

If the main determinant of jawline width is too prominent a mandible bone, injecting BOTOX will make no difference.

The effect of muscle injections can vary widely, from almost undetectable to undesirably extreme. If the masseter muscle is not overly developed to start, BOTOX injections may simply shrink the muscle enough to make the prominent bone underneath it appear even more objectionable.

Eliminating behaviors such as teeth clenching or constant gum chewing that may stimulate muscular development may also be of some value.

BOTOX has also been reported to shrink an enlarged parotid gland (salivary gland) present underneath the masseter muscle without affecting saliva production. While this can magnify the effect of jawline slimming, whether it is a safe to do so repeatedly is another question.