What must be marked on drug samples provided to physicians?

Drug samples that are provided to physicians must be marked with "Sample" or "Not for sale" to ensure clarity regarding their purpose and limitation on distribution. This labeling is crucial as it indicates that these drugs are not intended for resale and is part of regulatory requirements designed to prevent misuse or illegal distribution of pharmaceuticals. By marking samples in this manner, it underscores the ethical and legal obligations of both the pharmaceutical companies providing the samples and the healthcare providers receiving them, emphasizing that these samples are only meant to support patient care and are not part of commercial transactions.

The other options do not properly align with the regulatory requirements for drug samples. For instance, marking a sample as "For resale only" or "Private use only" could mislead providers regarding the intended use of the drug, while the statement "Prescription required" typically pertains to pharmaceuticals that must be dispensed through a licensed pharmacy and not to samples provided directly to physicians. Therefore, marking as "Sample" or "Not for sale" is the correct and regulatory-compliant method of labeling drug samples.