Botox Injection for Uneven Masseter Muscles? How Many Units Do I Need? (photo)

Hi, I have never had any botox injections before so Im a little concerned. My face is a little bit assymetrical along the jawline. At first I thought it was my bone shape but then I realized that it was my masseter muscle causing the assymetry. What are the side effects of botox? How many units do I require? Also, Im a bit concerned about getting injections in my muscle every year, will there be any long term effects? The photos do not look that bad, but in person the assymetry is more obvious.