Which of the following types of drugs can be purchased without a prescription?

OTC drugs, or over-the-counter drugs, are specifically designed for purchase without a prescription. This means that they can be safely used by the public for common health concerns without needing the guidance of a healthcare professional. These medications are generally considered safe and effective for self-medication when consumers follow the instructions provided on the packaging.

In contrast, legend drugs and Rx drugs refer to the same category of medications that require a prescription from a licensed healthcare provider. These drugs are typically used for more serious health conditions and necessitate professional supervision due to their potency, potential side effects, or the complexity of their use.

Controlled substances are a subset of drugs that are regulated due to their potential for abuse, addiction, and dependence. These substances typically cannot be obtained without a prescription and often have stricter control measures in place compared to other prescription medications.

Thus, OTC drugs are the category among these options that provides the convenience of being available without a prescription.