What warning must be included on a prescription label?

The requirement for including a specific warning on a prescription label is primarily focused on ensuring patient safety and providing crucial information about the potential risks associated with the medication. The selected answer emphasizes the importance of consulting a healthcare provider before using the medication in conjunction with alcohol or other non-prescribed drugs. This warning is significant because many medications can interact adversely with alcohol or other substances, potentially leading to harmful side effects or reducing the efficacy of the drug. By advising the patient to consult with the prescribing practitioner, the label promotes safer medication practices.

In contrast, while the other options contain important safety warnings, they are generally less comprehensive or may not be universally required on all prescription labels depending on the specific medication. For instance, caution about drowsiness or driving might be relevant for certain medications but not applicable to all. Similarly, designating a product for external use is specific to certain formulations and may not be relevant to prescription medications intended for systemic use. Thus, the selected answer encompasses a broader principle of safety and responsible use of medication that is most pertinent for a prescription label.