A prescription drug is a approved medicine that is regulated by legislation to need a prescription before it can be obtained. The term is used to distinguish it from over-the-counter drugs which can be obtained without a prescription. Different jurisdictions have dissimilar definitions of what constitutes a prescription drug. As a universal rule, over the counter drugs are used to luxury conditions not necessarily requiring a doctor's care and will have been established to meet higher security standards for self-medication by patients. Often a inferior dosage of a drug will be approved for OTC use, while higher dosages will stay the province of a doctor's prescription; a prominent case is ibuprofen, which has been extensively available as an OTC pain killer since the mid-1980s but is still obtainable in doses up to four times the OTC dose for use in cases of severe orthopedic pain.

In the United States, the word "prescription drug" is most normally used, but they are also called legend drugs or Rx-only drugs, after the supplies of Federal and state laws that all such drugs bear a "legend" keeping out sale without a prescription; though more multifaceted legends have been used, on most unique drug packaging today the legend just says "Rx only". In the United Kingdom, they are referred to as Prescription Only Medicine or POM.

In the United States, the Federal Food, Drug, and Cosmetic Act describe what requires a prescription. Prescription drugs are usually authorized by doctors, though physician assistants and nurse practitioners do a rising amount of drug prescribing. It is usually required that an MD, DO, PA or NP write the prescription; nurses, tragedy medical technicians, psychologists (but not psychiatrists, who are MDs), as examples, do not usually have the authority to prescribe drugs. Unlike several other countries, the United States does not have cost controls for prescription drugs, and US drug prices are often apparent as inflated in contrast to other countries; therefore, most health insurance programs (normally partially or in full paid for by the patient's employer) have instruction payment plans where the patient pays only a small co-payment and the pharmacy is reimbursed for the break of the cost by the insurance company.