Will eating after Botox on masseter decrease the effect and can you do another injection in 3 weeks?

Botox injections are done in the masseters either to decrease bruxism (teeth grinding) or in order to decrease muscle hypertrophy. Normal chewing may increase the uptake of the medication but excessive chewing will overwork the muscles and may lead to hypertrophy (a normal reaction of muscles to repeated and increased work load). Bruxism will imrpove in few days as Botox takes effect. Visible reduction of the masseter size will take much longer and is noticeable after 8 weeks or longer. 

Dear iamamermaid:
Chewing immediately after Botox is injected works the masseter muscles and at least theoretically may increase the uptake of Botox by the neuromuscular junctions such that more Botox is taken up and less diffuses away.  But excessive chewing, such as chewing gum for hours a day, may actually increase the size of the masseter muscles,
Usually an effect is seen by one week, but if your muscles are very large, you may not see much benefit from your first treatment, in which case more should be injected 7 to 14 days later.  As you continue your treatments (approximately every four months), you will see more of an effect from the same dose. Therefore, the dose may actually be reduced over time.
All the best,   Dr. Clark