What happens to the leftover Botox?

What is done with the remaining Botox is a question only your doctor can answer. Once reconstituted the product can be used for up to 24 hours, longer in some situations if stored properly. Your should only be charged for the units that were injected.

Every Botox vial has a lot number.  Doctors are to keep an accurate log of which botox/lot number was used on each patient.  If a problem arises and the medical records are audited, the Botox vial numbers/lot numbers better match up, otherwise the physician can face a lot of legal consequences and problems.  I personally don't think it's worth the risk for anyone to try to cheat the system.  The insurance companies expect the unused Botox units to be discarded; if used on other patients, it's considered a form of stealing.  I hope your physician is more careful and ethical than to do that. 

Namebrand BOTOX comes in either 50 U vials or 100 U vials.  It is a very expensive drug for doctors to buy, since it typically costs about $530 for a 100 U vial.   Because of the cost of BOTOX, it is typically never thrown away since it has many uses and can be refrigerated and will keep at least a week.  You pose a very interesting question.