Which characteristic is essential for a narcotic?

A narcotic, in the context of pharmacology, is primarily defined by its ability to relieve pain. This characteristic is crucial because narcotics, often classified as opioids, are used therapeutically to manage moderate to severe pain. They work by binding to specific receptors in the brain and spinal cord, which helps to decrease the perception of pain and can also produce feelings of euphoria.

The other choices do not correctly define a characteristic of narcotics. Non-addictiveness is not a hallmark of narcotics; in fact, many narcotics are known for their potential to cause addiction. The notion that they must always be derived from plants is also misleading, as modern narcotics can be both naturally derived and synthetically manufactured. Lastly, the requirement that they can only be prescribed by specialists is not accurate; while there are regulations around prescribing narcotics, many healthcare providers, including general practitioners, are authorized to prescribe them when deemed appropriate. Thus, the ability to relieve pain stands out as the essential characteristic of narcotics.