When is it unnecessary to use a red C on prescriptions?

Using a red C on prescriptions is specifically required for controlled substances to indicate their status, which helps in identifying and tracking these medications due to their potential for abuse and dependence. When a computer system can identify the prescription by number, it indicates that the prescription has already been categorized and tracked electronically, minimizing the need for additional physical markings like a red C.

This electronic identification ensures that the prescription is recognized within the system and that all necessary regulatory requirements are being met without the need for manual markings. Therefore, in such instances where the computerized system can clearly and effectively manage the prescriptions, it becomes unnecessary to use the red C.

The other options do imply scenarios where the red C may not be required, but they do not hold the rationale that stems from the operational efficiency and tracking capabilities provided by electronic prescription systems.