Which of the following substances are considered depressants?

The correct choice identifies barbiturates and alcohol as depressants. Depressants are substances that reduce the activity of the central nervous system, leading to a calming effect. They can lower anxiety levels, slow down brain function, and in higher amounts can impair motor function and judgment.

Barbiturates, which are sedative medications, work by enhancing the effects of the neurotransmitter gamma-aminobutyric acid (GABA), promoting relaxation and decreased neural activity. Alcohol, on the other hand, also acts as a depressant by inhibiting the release of certain neurotransmitters, resulting in similar effects on relaxation and reduced inhibition.

Other options mention substances that are categorized differently. Stimulants and analgesics, for example, stimulate the central nervous system rather than calm it. Amphetamines are stimulant drugs that increase alertness and energy. Analgesics, typically known for pain relief, can include both depressants (like opioids) and non-depressants (like ibuprofen). Hallucinogens also serve a different purpose, often altering perception and mood rather than causing central nervous system depression.