I have had Botox twice on my forehead and on both occasions only the left side has worked. Why would this happen?

This is very unusual. Typically, this is due to either underdosing that side with Botox or injecting the product incorrectly. If it worked on one side but not the other the most likely explanation is that you need additional units on the stronger side. Your nerve endings are the same throughout the facial muscles and if Botox works on one side it will work similarly on the other side. Best.Stephen Weber MD, FACSDenver Facial Plastic Surgeon

Thank you for your question.  Muscle are asymmetric. Maybe your forehead muscles are thicker and require more units of botox.  This should of been corrected the second time,. You did not mention how many units of botox you received.  You might of been under dosed.  Maybe try a board certified injector. Best Wishes.