Sec. 20. (a) Except as provided in section 30 of this chapter, a prescription or drug order for a legend drug is not valid unless the prescription or drug order is issued for a legitimate medical purpose by a practitioner acting in the usual course of the practitioner's business.
(b) A practitioner may not knowingly issue an invalid prescription or drug order for a legend drug.
(c) A pharmacist may not knowingly fill an invalid prescription or drug order for a legend drug.