Which of the following substances is NOT classified as a non-narcotic depressant?

Heroin is classified as a narcotic depressant, not a non-narcotic depressant. Narcotics, also known as opioids, primarily act on the central nervous system to produce analgesia and sedation. They are derived from the opium poppy or synthesized to mimic its effects. Heroin specifically binds to opioid receptors in the brain, which is why it causes intense feelings of euphoria followed by sedation.

In contrast, the other substances listed—tranquilizers, alcohol, and barbiturates—are categorized as non-narcotic depressants. Tranquilizers, such as benzodiazepines, are used to relieve anxiety and induce sedation but do not possess the same properties as narcotics. Alcohol is a widely known depressant that affects mood and cognitive functions, while barbiturates are prescribed for their sedative effects. Since the question asks for a substance that is NOT classified as a non-narcotic depressant, heroin stands out as the correct answer.