Which of the following drugs is a tricyclic antidepressant?

Doxepin is classified as a tricyclic antidepressant (TCA) because it is part of a group of medications that share a common chemical structure characterized by three interconnected rings. TCAs are primarily used in the treatment of depression but can also be effective for anxiety disorders, chronic pain, and insomnia. They work by inhibiting the reuptake of norepinephrine and serotonin, neurotransmitters that play a crucial role in mood regulation.

In contrast, the other options are not classified as tricyclic antidepressants. Diazepam is a benzodiazepine used for anxiety, muscle spasms, and seizures. Cocaine is a stimulant and local anesthetic that acts on the central nervous system but does not fall under the category of antidepressants. Phenobarbital is a barbiturate primarily used for seizure control and is not related to the structural or functional characteristics of tricyclic antidepressants.