Botulinum toxin type A (Botox) only comes in 100 unit vials that must be reconstituted prior to use. After it is reconstituted, it has an approximately "self-life" of 4 hours. Providers can split vials between patients, i.e. one patient is injected with 125 units and another patient is injected with 175 units. Each patient would be billed for the specific amount injected, i.e. first patient billed J0585 125 units and second patient is billed J0585 175 units.
If there isn't any patients to split a vial with and the provider must unavoidably waste the balance, the patient can be billed for the unavoidable wasteage. For example, only one patient who is injected with 175 units of Botulinum toxin A. There are no other patients scheduled on same day that need chemodenervation. The provider wastes the 25 units (the balance of the second vial), documents the amount of the wastage and the reason for the wastage (no other patients medically need chemodenervation) and then the patient is billed for the 200 units.
Most payers allow providers to report wastage on the same line item as the amount injected, i.e.
J0585 200 units
Exception is Trailblazer Medicare wants the wastage billed on a separate line item with the JW modifier appended:
J0585 175 units
J0585 -JW 25 units
FYI: make sure that you have pre-authorization for both the injection code (64612 - 64614) AND the drug (J0508 or J0587). Many payers have very limited coverage policies for botulinum toxin injections, especially for pain management. Both drugs are quite expensive especially if the insurance does not cover the particular condition.