Diffrence between 20 unit botox and 40 unit?

I suggest you use the number of units needed for the type of muscle and shape you have. I suggest speaking to your physician for the best number.  Best, Dr. Emer.

If the 40 units is a better deal, go for it!!!  You may not need at a single treatment but use the remainder in a few months, if that is allowed.  Your injector should be able to inform you about how much you will need during a single injection visit and let that guide your decision.  Best of luck.