What must all legend prescriptions include?

The correct choice emphasizes a critical legal requirement for legend drugs in the United States. The phrase "Caution: federal law prohibits dispensing without a prescription" reflects the mandate that these medications, also known as prescription drugs, cannot be dispensed without a valid prescription from a licensed healthcare provider. This warning serves to inform both patients and pharmacists about the legal framework governing the distribution of these medications, thereby ensuring the drugs are used safely and appropriately.

This phrase is mandated by federal law, specifically by the Food, Drug, and Cosmetic Act, which defines how prescription medications are to be dispensed and reinforces the need for professional oversight. This message is essential for preventing misuse and ensuring that these medications are taken under medical supervision, which is crucial for patient safety.

While other statements may provide valuable information or indications regarding drug use, they do not carry the same level of legal significance or direct communication about the necessity of a prescription for dispensing, which is why they are not the correct answer in this context.