A legend drug is a drug that can only be procured by prescription, which can only be gained from a physician authorized to prescribe medications.
There are certain types of medications that can be sold to anyone over the counter or to anyone over a certain age, as well as medications that can be dispensed by nurses or other health practitioners working within a medical center or doctor's office. The category of medication that needs to have the assessment of a trained practitioner before enacting the final determination to dispense is highly controlled.
The only way to procure these type of medications is to take an authorized prescription, i.e. one that is signed by the doctor, to a pharmacy for a certified pharmacist to fill. Sometimes, the doctors also have samples of medications that they are authorized to dispense, and these would be considered legend drugs.
The reason why there is a category of medication called legend drugs is because there is a legal requirement for physicians to only prescribe medications that are necessary after examining patients and to record all medications given to any patient in any regard. Furthermore, many of these types of medications (such as anti-depressants or pain medication) are habit-forming, so physicians are required by law to ensure that there are no more than a set legal limit prescribed to any person over any given time frame. The determination of the amount of medication that can be prescribed is determined by a national authority, which has reviewed all related studies in relation to each medicine, and who also holds the qualifications to assess medications.
If a physician prescribes a legend drug in a quantity (dosage) that is more than a person should be able to take when considering their age, size and past medical history, they can be sued for medical malpractice, which is why all physicians are forced to hold malpractice insurance. If a patient feels negative side effects or experiences long-term damage as the result of a prescription that was prescribed in error, the doctor and medical facility could be held liable. In order to ensure that patients do not take medicines that could counteract each other, a solid record of information needs to be kept—one that records each medication taken. This is also a reason why legend drugs exist.
In order to circumvent the legend drug category, patients who are unable to gain the prescriptions that they desire from their physicians, for whatever reason, sometimes try to buy the prescription online. This is illegal in the U.S. and in most other countries.