Office Drug Billing Question


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I have encountered a new scenario and I would like to know if anyone else has experienced this or if anyone can guide me to the correct area to find if this is acceptable or not.

Here is the scenario:
Dr. A has an office and he asked Dr. B to come and perform pain management procedures for his patients only. Dr. B works out of a different hospital, but has agreed to do these procedures at Dr. A's office. Dr. A purchased some drugs/supplies and told Dr. B that he can purchase any drugs/supplies needed from him to perform his services.

My question is: How can Dr. B bill for these drugs/supplies if he is purchasing them from Dr. A and not a drug/supply company?

Any guidance is greatly appreciated!
Thank You!