Which drug is classified as a tricyclic antidepressant?

Doxepin Hydrochloride is classified as a tricyclic antidepressant (TCA) due to its chemical structure and mechanism of action. TCAs are characterized by their three-ring structure, which is integral to their ability to inhibit the reuptake of neurotransmitters like serotonin and norepinephrine in the brain. This action helps improve mood and alleviate symptoms of depression.

In the context of the other choices, Phentermine Hydrochloride is an appetite suppressant used primarily for weight loss and does not have antidepressant effects. Varenicline Tartrate is primarily used for smoking cessation by targeting nicotinic receptors in the brain, making it distinctly different from antidepressants. Theophylline Anhydrous is a medication primarily used to treat respiratory diseases such as asthma and COPD by relaxing the muscles of the airways. Its mechanism and therapeutic uses are unrelated to those of tricyclic antidepressants. Thus, Doxepin Hydrochloride stands out as the correct answer for its classification and use in treating depression.