Final answer:
A valid prescription is one that is issued for a legitimate medical purpose by a prescriber acting in the usual course of their professional practice, following proper medical evaluations and legal requirements.
Step-by-step explanation:
To be considered a valid prescription, it must have been issued for a legitimate medical purpose by a prescriber acting in the usual course of their professional practice. There are important regulations and standards that govern the issuance of prescriptions to ensure that the process is used for valid therapeutic reasons and to minimize the misuse of medications, especially those that are controlled substances. The prescriber must conduct an appropriate medical evaluation, establish a diagnosis, and select the correct medication and dosage for the patient's condition. Furthermore, the prescriber must maintain proper records and adhere to applicable legal requirements.