Which of the following legends must a dangerous drug bear?

The designation "Rx only" is required on a dangerous drug to indicate that it can only be dispensed to patients with a valid prescription written by a licensed healthcare provider. This label is crucial because it signals to pharmacists and patients alike that the drug has potential risks or side effects that necessitate professional oversight in its use. It ensures that the drug is used appropriately, reducing the risk of misuse or adverse reactions that could arise from self-medication.

In contrast, the other options do not convey necessary regulatory information. For instance, stating that a drug can be dispensed without a prescription contradicts the safety protocols for dangerous drugs. Claiming that a drug is "free from side effects" is misleading because all medications can have potential side effects and contraindications, making it an inappropriate assertion. Lastly, marking a drug as "for personal use only" does not provide clarity on the requirements for its dispensing and does not align with the necessary supervision implied by the prescription requirement.